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Wave

Wave

:This article is about waves in the most general scientific sense; a separate article focuses on ocean waves. For other meanings see wave (disambiguation). Soundwave redirects here. for the Transformers character, see Soundwave (Transformers) A wave is a disturbance that propagates in a periodically repeating fashion, often transferring energy. A mechanical wave exists in a medium (which on deformation is capable of producing elastic restoring forces) through which they travel and can transfer energy from one place to another without any of the particles of the medium being displaced permanently; there is no associated mass transport. Instead, any particular point oscillates around a fixed position. However, electromagnetic radiation, and probably gravitational radiation are not mechanical waves, and can travel through a vacuum, without a medium. Waves are characterised by crests (highs) and troughs (lows), either perpendicular (in the case of transverse waves) or parallel (in the case of longitudinal waves) to wave motion.
The medium which carries a wave
A medium that can carry a wave is classified by one or more of the following properties:
- A linear medium if the amplitudes of different waves at any particular point in the medium can be added.
- A bounded medium if it is finite in extent, otherwise unbounded.
- A uniform medium if its physical properties are unchanged at different locations in space.
- An isotropic medium if its physical properties are the same in different directions.
Examples of waves
troughs
- Ocean surface waves, which are perturbations that propagate through water (see also surfing and tsunami).
- Visible light, radio waves, x-rays, gamma rays, infrared rays, and ultraviolet rays make up electromagnetic radiation. In this case propagation is possible without a medium, through vacuum. These electromagnetic waves travel at about 300,000 km/s.
- Sound - a mechanical wave that propagates through air, liquid or solids, and is of a frequency detected by the auditory system. Similar are seismic waves in earthquakes, of which there are the S, P and L kinds.
- Gravitational waves, which are fluctuations in the gravitational field predicted by General relativity. These waves are nonlinear.
Characteristic properties
All waves have common behaviour under a number of standard situations. All waves can experience the following:
- Reflection – the change of direction of waves, due to hitting a reflective surface.
- Refraction – the change of direction of a wave due to them entering a new medium.
- Diffraction – the spreading out of waves, for example when they travel through a small slit.
- Interference – the superposition of two waves that come into contact with each other.
- Dispersion – the splitting up of waves by frequency.
- Rectilinear propagation – the movement of waves in straight lines.
Transverse and longitudinal waves
Rectilinear propagation Transverse waves are those with vibrations perpendicular to the direction of the propagation of the wave; examples include waves on a string and electromagnetic waves. Longitudinal waves are those with vibrations parallel to the direction of the propagation of the wave; examples include most sound waves. Ripples on the surface of a pond are actually a combination of transverse and longitudinal waves; therefore, the points on the surface follow elliptical paths.
Polarization
Transverse waves can be polarized. Unpolarised waves can oscillate in any direction in the plane perpendicular to the direction of travel, while polarized waves oscillate in only one direction perpendicular to the line of travel.
Physical description of a wave
Image:wave.png Waves can be described using a number of standard variables including: frequency, wavelength, amplitude and period. The amplitude of a wave is the measure of the magnitude of the maximum disturbance in the medium during one wave cycle, and is measured in units depending on the type of wave. For examples, waves on a string have an amplitude expressed as a distance (meters), sound waves as pressure (pascals) and electromagnetic waves as the amplitude of the electric field (volts/meter). The amplitude may be constant (in which case the wave is a c.w. or continuous wave) or may vary with time and/or position. The form of the variation of amplitude is called the envelope of the wave. The period (T) is the time for one complete cycle for an oscillation of a wave. The frequency (F) is how many periods per unit time (for example one second) and is measured in hertz. These are related by: : $f=\frac$ When waves are expressed mathematically, the angular frequency (ω, radians/second) is often used; it is related to the frequency f by: : $f=\frac$ .
Travelling waves
Waves that remain in one place are called standing waves - e.g. vibrations on a violin string. Waves that are moving are called travelling waves, and have a disturbance that varies both with time t and distance z. This can be expressed mathematically as: : $y=A(z,t) \cos (\omega t - kz + \phi),\,f$ where A(z, t) is the amplitude envelope of the wave, k is the wave number and φ is the phase. The velocity v of this wave is given by: : $v=\frac= \lambda f,$ where λ is the wavelength of the wave.
Propagation through strings
The speed of a wave travelling along a string (v) is directly proportional to the square root of the tension (T) over the linear density (ρ): : $v=\sqrt.$ This equation can be found using dimensional analysis
The wave equation
The wave equation is a differential equation which describes a harmonic wave passing through a medium, discussed above. The equation has different forms depending on how the wave is transmitted, and on what medium. Not all waves are sinusoidal. One example of a non-sinusoidal wave is a pulse that travels down a rope resting on the ground, extending in direction x, travelling at velocity c. The height of the pulse above the ground is φ. The distance the pulse travels between some time t and time 0 is ct. In one dimension the wave equation has the form : $\frac\frac=\frac. \$ A general solution, given by d'Alembert is : $\phi(x,t)=F(x-ct)+G(x+ct). \$ considered to be the shapes of two pulses travelling down the rope, F in the +x direction, and G in the -x direction. If we substitute for x above, instead directions x, y, z, we then can describe a wave propagating in three dimensions. A non-linear wave-equation can cause mass transport. The Schrödinger equation describes the wave-like behaviour of particles in quantum mechanics. Solutions of this equation are wave functions which can be used to describe the probability density of a particle. Quantum mechanics also describes particle properties that other waves, such as light and sound, have on the atomic scale and below.
External links

- [http://www.lightandmatter.com/area1book3.html Vibrations and Waves] - an online textbook
- [http://kestrel.nmt.edu/~raymond/classes/ph13xbook/node1.html A Radically Modern Approach to Introductory Physics] - an online physics textbook that starts with waves rather than mechanics
See also

- List of wave topics
- Capillary waves
- Doppler effect
- Group velocity
- Phase velocity
- Ripple tank
- Standing wave
- Audience wave
- Ocean surface wave
- Waving Category:Partial differential equations ko:파동 ms:Gelombang ja:波動 simple:Wave

Ocean surface wave
] right Ocean surface waves are surface waves which occur at the surface of an ocean. That is, a wave that is guided along the interface between water and air. As the wind blows, pressure and friction forces associated with the wind perturb the equilibrium of the ocean surface. The wind actually transfers some of its energy into the water. The water is able to gain energy from the wind because of the friction between the wind and the water. This causes the surface particles to move in an elliptical motion, which is a combination of longitudinal (back and forth) and transverse (up and down) wave motions. A good illustration of the wave motion is given by
- [http://www.coastal.udel.edu/faculty/rad/linearplot.html Prof. Robert Dalrymple Java applet]. transverse As the depth into the ocean increases, the radius of the elliptical motion decreases. At a depth equal to half the wavelength λ, the orbital movement is zero. The speed of the surface wave is also called celerity or phase velocity because it corresponds the speed of the shape of the wave, but is different from the speed of the water particles. This celerity is well approximated by : $c=\sqrt$ where : $c$ = phase speed in m/s; : $\lambda$ = wavelength in m; : $d$ = water depth in m; : $g$ = gee in m/s^2. In deep water, where $d \ge \frac\lambda$ , $s$ approximates $1.25 \sqrt\lambda$ . This expression tells us that waves of different wavelengths travel at different speeds: waves disperse. The fastest waves in a storm are the ones with the longest wavelength. As a result, when waves a arrive on the coast from a storm the first ones to arrive are the long swells. When several wave trains are present, which is always the case in the ocean, the waves form groups. In deep water the groups travel at a group velocity which is half of the phase velocity. Following a single wave in a group one can seen the wave appearing at the back of the group, growing and finally disappearing at the front of the group. As the water depth $d$ decreases towards the coast, this will have an effect on the speed of the crest and the trough of the wave; the crest moves faster than the trough. This causes surf, a breaking of the waves. Individual "freak waves" (also "rogue waves", "monster waves" and "king waves") sometimes occur in the ocean, often as high as 30 metres. Such waves are distinct from tides, caused by the moon and sun's pull, and tsunamis that are caused by underwater earthquakes or landslides. The movement of ocean waves can be captured by wave energy devices. The energy contained in 1 m² of sea depends on the water density $\gamma$ and the wave height h (= 2·amplitude ( $y$ ): : $E=\frac\gamma^2=\frac\gamma y^2.$
Types of waves
amplitude When waves "break", they have different characteristics depending on factors including the structure of the ocean floor. There are three main types that are identified by surfers or surf lifesavers: plunging waves (also known as "dumpers"), spilling waves and surging waves. Their varying characteristics make them more or less suitable for surfing and present different dangers.
External links

- [http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10 Introductory oceanography chapter 10 - Ocean Waves]
- [http://hyperphysics.phy-astr.gsu.edu/hbase/waves/watwav2.html HyperPhysics - Ocean Waves]
- [http://www.shom.fr/fr_page/fr_act_oceano/vagues/vagues_f.htm SHOM - in French] Category:Physical oceanography Category:Water waves ja:水面波

Transformers (toyline)

Transformers is the name of a line of toys produced by Hasbro from 1984 onwards, and also of a number of spin-offs based on the toys including a Marvel comic book series, an animated television series that began airing in 1984 (Transformers series) and a feature-length movie, Transformers: The Movie. The original series was followed by a number of spin-offs with varying levels of popularity. A Transformer is an intelligent robot (usually a large humanoid, though there are many exceptions such as animal forms) that is able to "transform", reconfiguring itself into a common and innocuous form, such as a car, airplane or animal. The taglines "More Than Meets the Eye" and "Robots in Disguise" reflect this ability. Transformers originally featured two main factions warring for control of their home planet, Cybertron. The heroic Autobots (Cybertrons in the Japanese version) were led by Optimus Prime, and their opponents, the Decepticons (Destrons in the Japanese version), were led by Megatron. The Autobots were mainly cars in warm colors while the Decepticons were planes in cool colors, with some exceptions. The Transformers toyline was developed by Hasbro after they met up with Takara representatives at the 1983 New York Toyfair and decided to combine and re-brand Takara's Diaclone and Micro Change toylines into the Transformers for release in the United States. The basic backstory of the toyline and subsequent comic books and cartoons was developed by the Marvel Comics writers Jim Shooter and Dennis O'Neil (O'Neil actually giving Optimus Prime his name). Most of the subsequent character names and profiles throughout the original run were done by the primary Transformers US comic book writer, Bob Budiansky. Floro Dery was primarily responsible for the look and feel of the Transformers cartoon series and was the visual creator of Transformers: The Movie. He refined some of the initial season one animated character models done in Japan, and subsequently interpreted the toy box art for further characters, creating the models that would become the visual guidelines both for the comic books and the animated cartoon. Most Transformers come with tech specs which detail the Transformer’s characteristics. Older Transformers come with Robot Points which could be redeemed for special Transformers which were not sold in stores, such as the Omnibots or the Decepticon triplets known as Reflector (which featured heavily in the early episodes of the television series, despite not being easily available as toys).

Incarnations

The following Transformers toys came out:
- Transformers (1984-1990) - retroactively called Generation 1 or G1 since then.
- Transformers (1984-1992) - Japan and UK series ran longer than US.
- Transformers: Generation 2 (1992-1995)
- Beast Wars: Transformers (1995-1999)
- Machine Wars: Transformers (1997) - a limited release Kay Bee exclusive
- Beast Machines: Transformers (2000-2001)
- Transformers: Robots in Disguise (2001-2002)
- Transformers G1 Commemorative Series (2002-2005)
- Transformers: Armada (2002-2003)
- Transformers: Universe (2003-present)
- Transformers: Energon (2003-2005)
- Transformers: Alternators (2003-Present)
- Transformers: Cybertron (2005-Present) See also: Transformers Universes and Transformers series.

Transformers (Generation 1) (1984-1992)

The first Transformers toys were brought together from the different transforming robot toylines from Takara, notably the Diaclone and Micro Change (Micro Man) series. Hasbro acquired the rights to reproduce them in the United States but instead of selling them as their original names, they were rebranded as "Transformers". The first two years consisted primarily of reusing the Diaclone/Micro Change molds. Some of the models from the Diaclone line still have the pilot's seat in their design. The tagline to the Transformers is "More than meets the eye!" It was in 1986, the third year, when Hasbro began designing new original models. It was also the time when subgroup Transformers became more popular than simply labeling a character as Autobot or Decepticon. There were the Aerialbot group, Dinobot group, Predacons, Headmasters and so on. This trend continued on until the toyline's demise in 1990. In 1989, the entire line became limited to Pretenders and Micromasters. For the first time, Transformers received a new design for their title logo. But this was also regarded by many as a time of a dearth in creativity and regarded as the lowest point in the toyline's history. 1990 saw the last American burst with the release of more Micromaster characters and the introduction of the Action Masters, Transformers who can't transform. The Action Master line was criticized although it had a few defenders. This would be the last Transformers output in the US until 1992. While Transformers ended poorly for the US market, the same can not be said for the UK and Japan markets as they went on to produce their own continuing series between 1991 to 1992. Each country produced their own continuity. The UK continued with new Action Master figures and introduced the Turbo Masters and Predators. Japan continued with the Micromasters concept.

Transformers: Generation 2 (1992-1995)

In late 1992, Hasbro relaunched the Transformers franchise with the Generation 2 line. The subgroups concept is done away with for the first year but there are no new molds or characters. Hasbro re-used the molds for most of the characters from the 1984 and 1985 line but with mostly different color schemes and finishes as well as different weapons and accessories. Megatron's figure was released later on. Megatron's original alternate mode was a gun but in Generation 2 this is changed to a tank due to safety and security concerns. This line was criticized for the poor material used and being easily breakable. Generation 2 sold poorly and was abandoned by Hasbro after two years.

Beast Wars/Machines (1995-2001)

With the failure of the Generation 2 series, Hasbro decided the franchise needed an overhaul. They went in a new direction, a new beginning. Instead of robots disguising themselves as cars and planes, the idea is now of robots transforming into animals. While there have been Transformers before that change into animals, the idea here is they all change into real-looking animals. Robots on the inside, flesh on the outside. The Beast Wars toyline is launched in the fall of 1995. A CGI animated series produced by Mainframe Entertainment was aired to tie-in with the new toyline. A fresh idea coupled with a TV series with strong stories assured this series the much needed success Hasbro needed. Beast Wars was renamed in some countries, particularly Canada, because of concern over the word "war" in the title. So, in some countries, it was released as Beasties. Long-time fans notice the prominence of the words "Beast Wars" over "Transformers", the latter appearing in small type under the former. Two fan groups formed with one enjoying Beast Wars for what it is and another who do not think it should not be part of the Transformers mythology. The success of Beast Wars and a change in storyline resulted in its second phase: Beast Machines. Like Beast Wars, the name Transformers is used only as a secondary title. While still a success, the storyline and direction borne by Beast Machines was questioned and criticized by the most ardent fans who knew the previous history of Transformers. Also, there was a clamor for a return to the original idea of vehicle-changing Transformers.

Backstory

(Note: This only applies to original G1 cartoon continuity, and not to many of the later series.) Ravaged from the war between the Autobots and the Decepticons, Cybertron was almost completely drained of its energy resources. Neither side had enough energy reserves to continue the battle, which led to a stalemate. The Autobots, with their leader Optimus Prime, left their home planet to seek new sources of energy. The Decepticons pursue them and board the Autobot starship (called the Ark in the comic). During the ensuing fight, they crash-land into a volcano on prehistoric Earth. Awakened in 1984 when the volcano erupts, the Decepticons were repaired by the ship's computer and fled, leaving the still-deactivated Autobots behind. Decepticon leader Megatron soon discovered that Earth has nearly limitless energy resources. Hoping to tip the war's balance in favor of the Decepticons, Megatron planned to transfer Earth's energy to Cybertron even if it meant ruining the Earth in the process. Unfortunately for them, the Decepticons made a fatal mistake. After the Decepticons were awakened, the Autobots remained deactivated on the Ark. Upon leaving, the Decepticon Starscream simply blasted the rocks around the ship to seal the entrance, rather than destroying the helpless Autobots altogether; the jolt from the explosions moved Optimus Prime within the Ark's repair beam. The remaining Autobots are subsequently revived and rise up to become the protectors of life on Earth and the Decepticons' nemeses.

Trivia

- Contrary to what people may think, there has never been a break in the production of new Transformer toys; there have been new Transformers toys every year since its debut in 1984 because the UK and Japan produced their own continuing series in the period of 1992 to 1994.
- Like G.I. Joe's Larry Hama, Bob Budiansky wrote the majority of the tech specs (the personal profile of each Transformer) for the Generation 1 series.

External links

- [http://www.transformers.com/ Official Transformers Web Site]
- [http://www.bwtf.com/ Ben’s World of Transformers] - Features episode summaries and toy reviews.
- [http://www.thetransformers.net/ TheTransformers.Net] Europes largest and most popular Transformers Fan Site
- [http://www.tfmaster.com/ TFMaster Transformers Fan Site]
- [http://www.electric-escape.net/tf Rob’s Transformers Page] - synopses of Transformers comics, series guides to Japanese cartoons, news archives, and more.
- [http://www.geocities.com/Area51/Station/6563/ Stanley Lui’s Transformers On-Line Encyclopedia]
- [http://www.tfu.info/ TFU.Info] - Growing archive of nearly every Transformers toy from the beginning.
- [http://www.transformertoys.co.uk/ Transformers @ The Moon] - Most complete Transfomer toy image gallery on the web.
- [http://www.transformersnewzealand.com Transformers New Zealand.Com] Category:Hasbro products Category:Toys ja:トランスフォーマー

Medium (bearer)

Medium may mean:

General

- Recording medium
- Medium (spirituality), a person who claims to serve as an intermediary between the living and the dead
- Medium of instruction, the language used to educate in
- Average

Science

- Medium (optics)
- Processing medium in industrial engineering
- Transmission medium in physics
- Growth medium in biotechnology
- Interplanetary medium in astronomy
- Interstellar medium in astronomy
- Excitable medium
- Solvent in chemistry

Fiction

- Medium (TV series)
- The Medium, an opera by Gian Carlo Menotti

Electromagnetic radiation

Electromagnetic radiation is a propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation. The term electromagnetic radiation is also used as a synonym for electromagnetic waves in general, even if they are not radiating or travelling in free space. This sense includes, for example, light travelling through an optical fiber, or electrical energy travelling within a coaxial cable. Electromagnetic (EM) radiation carries energy and momentum which may be imparted when it interacts with matter.

Physics

Theory

Electromagnetic waves of much lower frequency than visible light were predicted by Maxwell's equations and subsequently discovered by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations which made explicit the wave nature of the electric and magnetic fields. These equations displayed the symmetry of the fields. According to the theory, a time-varying electric field generates a magnetic field and vice versa. Thus, an oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field, and so on. By this means an EM wave is produced which propagates through space.

Properties

Electric and magnetic fields exhibit the property of superposition. This means that the field due to a particular particle or time-varying electric or magnetic field adds to the fields due to other causes. (As magnetic and electric fields are vector fields, this is the vector addition of all the individual electric and magnetic field vectors.) As a result, EM radiation is influenced by various phenomena such as refraction and diffraction. For example, a travelling EM wave incident on a particular arrangement of atoms induces oscillation in the atoms and thus causes them to emit their own EM waves (called wavelets). These emissions interfere with the impinging wave and alter its form. In refraction, a wave moving from one medium to another of a different density changes its speed and direction when it enters the new medium. The ratio of the refractive indices of the media determines the extent of refraction. Refraction is the mechanism by which light disperses into a spectrum when it is shone through a prism. The physics of electromagnetic radiation is electrodynamics, a subfield of electromagnetism. EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). These characteristics are mutually exclusive and appear separately in different circumstances: the wave characteristics appear when EM radation is measured over relatively larger timescales and over larger distances, and the particle characteristics are evident when measuring smaller distances and timescales. EM radiation's behaviours as a wave and as a stream of particles have been confirmed by a large number of experiments.

Wave model

An important aspect of the wave nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, equal to one oscillation per second. Light usually comprises a spectrum of frequencies which sum to form the resultant wave. In addition, frequency affects properties like refraction, in which different frequencies undergo a different level of refraction. A wave has troughs and crests. The wavelength is the distance from crest to crest. Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. Frequency has an inverse relationship to the concept of wavelength. When waves travel from one medium to another, their frequency remains exactly the same - only their wavelength and/or speed changes. Waves can also be described by their radiant energy. Interference is the superposition of two or more waves resulting in a new wave pattern. The way that these coincide causes different types of interference.

Particle model

In the particle model of EM radiation, EM radiation is quantized as particles called photons. Quantisation of light represents the discrete packets of energy which constitute the radiation. The frequency of the radiation determines the magnitude of the energy of the particles. Moreover, these particles are emitted and absorbed by charged particles, so photons act as transporters of energy. A photon absorbed by an atom excites an electron and elevates it to a higher energy level. If the energy is great enough, the electron is liberated from the atom in a process called ionization. Conversely, an electron which descends to a lower energy level in an atom emits a photon of light equal to the energy difference. The energy levels of electrons in atoms are discrete. Therefore, each element has its own characteristic frequencies. Together these effects explain the absorption spectra of light. The dark bands in the spectrum are due to the atoms in the intervening medium which absorb different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, in a distant star, dark bands in the light it emits are due to the atoms in the atmosphere of the star. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum which represents the jumps between the energy levels of the electrons is exhibited. This is manifested in the emission spectrum of nebulae.

Speed of propagation

Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10^-34 J·s is Planck's constant, and ν is the frequency of the wave. One rule is always obeyed regardless of the circumstances. EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.)

Electromagnetic spectrum

Generally, EM radiation is classified by wavelength into electrical energy, radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. More in-depth information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, many hydrogen atoms emit radio waves which have a wavelength of 21.12 cm.

Light

EM radiation with a wavelength between 400 nm and 700 nm is detected by the human eye and perceived as visible light. If radiation having a frequency in the visible region of the EM spectrum shines on an object, say, a bowl of fruit, this results in our visual perception of identifying information from the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood "psychophysical phenomenon," most humans perceive a bowl of fruit. In the vast majority of cases, however, the information carried by light is not directly apprehensible by human senses. Natural sources produce EM radiation across the spectrum; so, too, can human technology manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data. Those data can be translated into sound or an image. The coded form of such data is similar to that used with radio waves.

Radio waves

Radio waves carry information by varying amplitude and by varying frequency within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens.

Derivation

Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. (For symbol definitions see magnetic field.) :

\nabla \cdot \mathbf = 0

\nabla \times \mathbf = -\frac \mathbf

\nabla \cdot \mathbf = 0

\nabla \times \mathbf = \mu_0 \epsilon_0 \frac \mathbf

\mathbf=\mathbf=\mathbf

is a solution, but there might be other solutions as well. Let us employ a useful identity from vector calculus. :

\nabla \times \left( \nabla \times \mathbf \right) = \nabla \left( \nabla \cdot \mathbf \right) - \nabla^2 \mathbf

Where

\mathbf

can be any vector function. Taking the curl of the curl equations and applying the identity, we get the following. :

\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf

\nabla^2 \mathbf = \mu_0 \epsilon_0 \frac \mathbf

These types of equations are identified as linear wave equations with wave speed

\frac

. Amazingly, this speed happens to be exactly the speed of light! Maxwell's equations have unified the permittivity of free space

\epsilon_0

, the permeability of free space

\mu_0

, and the speed of light itself:

c = \frac

. Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism. But these are only two equations and we started with four, so there is still more information pertaining to these waves hidden within Maxwell's equations. Let's consider a generic vector wave for the electric field. :

\mathbf = \mathbf_0 f\left( \hat \cdot \mathbf - c t \right)

Here

\mathbf_0

is the constant amplitude,

f

is any second differentiable function,

\hat

is a unit vector in the direction of propagation, and

\hat

is a position vector. We observe that

f\left( \hat \cdot \mathbf - c t \right)

is a generic solution to the wave equation. In other words :

\nabla^2 f\left( \hat \cdot \mathbf - c t \right) = \frac \frac f\left( \hat \cdot \mathbf - c t \right)

, for a generic wave traveling in the

\hat

direction. The proof of this is trivial. This form will satisfy the wave equation, but will it satisfy all of Maxwell's equations, and with what corresponding magnetic field? :

\nabla \cdot \mathbf = \hat \cdot \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = 0

\mathbf \cdot \hat = 0

The first of Maxell's equations implies that electric field is orthogonal to the direction the wave propagates. :

\nabla \times \mathbf = \hat \times \mathbf_0 f'\left( \hat \cdot \mathbf - c t \right) = -\frac \mathbf

\mathbf = \frac \hat \times \mathbf

The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of

\mathbf,\mathbf

. Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes,

\mathbf_0 = c \mathbf_0

. The electric field, magnetic field, and direction of wave propagation are all orthogonal and the wave propagates in the same direction as

\mathbf \times \mathbf

. Visualizing yourself as an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but you can rotate this picture around with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation, with respect to propagation direction, is known as polarization.

References

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External links

; General
- [http://www.sengpielaudio.com/calculator-wavelength.htm Conversion of frequency to wavelength and back - electromagnetic, radio and sound waves]
- [http://www.scienceofspectroscopy.info The Science of Spectroscopy - a learning tool for spectroscopy] ; Patents
- Greenleaf Whittier Pickard - - Intelligence intercommunication by magnetic wave component ko:전자기파 ja:電磁波

Vacuum

For other uses, see vacuum cleaner and Vacuum (musical group). The root of the word vacuum is the Latin word vacuum (pl. vacua) which means a space devoid of matter. In physics, a vacuum is the absence of matter in a volume of space.

Vacuum ranges

Vacuum ranges are defined as follows:

Perfect vacuum

A perfect vacuum is an ideal state that cannot practically be obtained in a laboratory, nor even in outer space, where there are a few hydrogen atoms per cubic centimeter at 10⁻¹⁴ pascal or 10⁻¹⁶ torr. In modern day usage vacuum is considered to exist in an enclosed space or chamber, when the pressure of gaseous environment is lower than atmospheric pressure (760 Torr or 101 kPa), or has been reduced as much as necessary to prevent the influence of some gas on a process being carried out in that space.

Partial vacuum

Physicists use the term partial vacuum to describe real-life non-ideal vacuum. A complete characterization of the physical state would require further parameters, such as temperature. The antithesis of a vacuum, which is also an ideal unachievable state, is called a plenum. In engineering, a vacuum is any region where the gas pressure is less than atmospheric pressure. Engineers measure the degree of vacuum in units of pressure. The SI unit of pressure is the pascal (abbreviation Pa), but vacuum is usually measured in millimeters of mercury (mmHg) or torr, with 1 mmHg or 1 torr equaling 133.3223684 pascals. It is often also measured using the barometric scale, or as a percentage of atmospheric pressure in bars or atms. For commercial purposes, vacuum is often measured in inches of mercury (inHg). This means that the pressure in vacuum, when specified in inches of mercury, is equal to the specified inches of mercury subtracted from 29.92. Thus a vacuum of 26 inHg is equivalent to a pressure of (29.92 - 26) or 3.92 inHg. Here, 29.92 inHg means perfect vacuum.

Degrees of vacuum

- Atmospheric pressure = variable, but standardised at 101.325 kPa (760 Torr) or 760 mm of mercury
- Vacuum cleaner = approximately 80 kPa (600 Torr)
- Mechanical vacuum pump = approximately 100 Pa to 100 μPa (1 Torr to 10⁻⁶ Torr)
- Near earth outer space = approximately 100 μPa (10⁻⁶ Torr)
- Cryopumped MBE chamber = 100 nPa to 1 nPa (10⁻⁹ Torr to 10⁻¹¹ Torr)
- Pressure on the Moon = approximately 1 nPa (10⁻¹¹ Torr)
- Interstellar space = approximately 1 fPa (10⁻¹⁷ Torr)
- [http://www-ssg.sr.unh.edu/ism/what1.html Source for interstellar vacuum] As gas pressure decreases, the mean free path (MFP) of the gas molecules increases. When the MFP is greater than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. In interplanetary and interstellar space, isotropic gas pressure is insignificant when compared to solar pressure, solar wind, and dynamic pressure. Astrophysicists prefer to use density to describe these environments, in units of particles per cubic metre.

Creating a vacuum

The easiest way to create an artificial vacuum is to expand the volume of a container. For example, your muscles expand your lungs to create a partial vacuum inside them, and air rushes in to fill the vacuum. By repeatedly closing off a compartment of the vacuum and exhausting it, it is possible to pump air out of a chamber of fixed size in a manner analogous to pumping a milkshake out of a glass. This is the principle behind most mechanical vacuum pumps. Inside the pump, a mechanism expands a small sealed cavity to create a deep vacuum. Because of the pressure differential, some air from the chamber is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size. A mechanical vacuum pump moves the same volume of gas with each cycle, but as the chamber's pressure drops, this volume contains less and less mass. So although the pumping speed remains constant when measured in litres/second, it drops exponentially when measured in kilograms/second. Meanwhile, the leakage rates, evaporation rates, and sublimation rates produce a constant mass flow into the system. When the pump's mass flow drops to the same level as the mass flows into the chamber, the system asymptotically approaches a constant pressure called the base pressure. Evaporation and sublimation into a vacuum is called outgassing, and the most common source is water absorbed by materials in the chamber. Outgassing can be reduced by desiccation prior to vacuum pumping. The base pressure of a rubber- and plastic-sealed piston pump system is typically 1 to 50 kPa, while a scroll pump might reach 10 Pa and a rotary vane oil pump with a clean and empty metallic chamber can easily achieve 0.1 Pa. If the dominant mass flow into the vacuum system is chamber leakage or outgassing of materials under vacuum, then the vacuum can be improved simply by installing bigger pumps. However, there is a point where backstream leakage through the pump and outgassing of the pump oils become the dominant mass flows into the chamber. In this situation, the vacuum will approach the pump's ultimate pressure - the best vacuum that this type of pump can achieve under ideal conditions. Adding more pumps in parallel or bigger pumps of the same type can still improve the pump-down speed, but they will not reduce the base pressure below ultimate. Better pumping technologies must be used to go beyond this barrier.

High vacuum

Fortunately, once the pressure has dropped below 1 kPa or so, another vacuum pumping technique becomes possible. Matter flows differently at different pressures based on the laws of fluid dynamics. At atmospheric pressure and mild vacuums, molecules interact with each other and push on their neighboring molecules in what is known as viscous flow. When the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and molecular pumping becomes more effective than compression pumping. This regime is generally called high vacuum. One such method to create a high vacuum to ultra high vacuum is by the use of cryopumps. Cryopumping incorporates the use of introducing cryogenics and a vacuum system. On a larger scale, the principles are the same as in a Cryomodule Molecular pumps sweep out a larger area than mechanical pumps, and do so more frequently, making them capable of much higher pumping speeds as measured in volume per time. They do this at the expense of the seal between the vacuum and their exhaust. Since there is no seal, a small pressure at the exhaust can easily force flow backstream through the pump; this is called stall. In high vacuum, however, pressure gradients have little effect on fluid flows, and molecular pumps can attain their full potential. The two main types of molecular pumps are the diffusion pump and the turbomolecular pump. Both types of pumps blow out gas molecules that diffuse into the pump. Diffusion pumps blow out molecules with jets of oil, while turbomolecular pumps use high speed fans. Both of these pumps will stall and fail to pump if exhausted directly to atmospheric pressure, so they must be exhausted to a lower grade vacuum created by a mechanical pump. As with mechanical pumps, the base pressure will be reached when leakage, outgassing, and backstreaming equal the pump speed, but now minimizing leakage and outgassing to a level comparable to backstreaming becomes much more difficult. High vacuum systems generally require metal chambers with metal O-ring seals such as Klein flanges or ISO flanges. The system must be clean and free of organic matter to minimize outgassing. All materials, solid or liquid, have a small vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. As a result, many materials that work well in low vacuums, such as epoxy, will become a problematic source of outgassing when attempting to achieve high vacuums. With these standard precautions, vacuums of 1 mPa are easily achieved with off-the-shelf molecular pumps. With careful design and operation, 1μPa is possible.

Ultra-high vacuum

:Main article: Ultra high vacuum Even higher vacuums are possible, but they generally require custom-built equipment, strict operational procedures, and a fair amount of trial-and-error. Yet more specialized pumps become useful: # Converting the molecules of gas to their solid phase by freezing them, called cryopumping or cryotrapping # Converting them to solids by electrically combining them with other materials, called ion pumping Ultra-high vacuum systems are usually made of stainless steel with metal-gasketed conflat flanges. The system is usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials in the system and boil them off. If necessary, this outgassing of the system can also be performed at room temperature, but this takes much more time. Once the bulk of the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures and minimize residual outgassing during actual operation. Some systems are cooled well below room temperature by liquid nitrogen to shut down residual outgassing and simultaneously cryopump the system. In ultra-high vacuum systems, some very odd leakage paths and outgassing sources must be considered. The water absorption of aluminium and palladium becomes an unacceptable source of outgassing, and even the absorptivity of hard metals such as stainless steel or titanium must be considered. Some oils and greases will boil off in extreme vacuums. The porosity of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face. The impact of molecular size must be considered. Smaller molecules can leak in more easily and are more easily absorbed by certain materials, and molecular pumps are less effective at pumping gases with lower molecular weights. Your system may be able to evacuate nitrogen, (the main component of air,) to the desired vacuum, but your chamber could still be full of residual atmospheric hydrogen and helium. Vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems. The lowest pressures currently achievable in laboratory are about 10^-13 Pa.

Vacuum in space

Pa Much of outer space has the density and pressure of an almost perfect vacuum. It is cold and has no friction. The properties of the vacuum remain largely unknown. A perfect vacuum is an ideal state that cannot practically be obtained in a laboratory, nor even in outer space, where there are a few hydrogen atoms per cubic centimeter at 10−14 pascal or 10−16 torr. All of the observable universe is also filled with large numbers of photons, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrinos. The current temperature is about 3 K, being merely 3 degrees above the absolute zero of temperature. Neither these photons nor the neutrinos produce a significant interaction with matter, so stars, planets and spacecraft move freely in this near perfect vacuum of interstellar space. Stars, planets and moons keep their atmosphere by gravitational attraction, so atmospheres have no firm boundary. The density of gas decreases with distance from the object. In Low Earth Orbit (about 300 km altitude) the atmospheric density is still sufficient to produce significant drag on satellites. Most Earth satellites operate in this region, and they need to fire their engines every few days to maintain orbit. The atmosphere in Low Earth Orbit is increasingly being polluted with man-made debris. Studies have discovered that some satellites retrieved from orbit are coated with a very thin layer of urine and fecal matter evidently released from Russian and US space missions. [http://see.msfc.nasa.gov/sparkman/Section_Docs/article_1.htm] Beyond planetary atmospheres, the pressure from photons and other particles from the sun become significant. Spacecraft can be buffeted by solar winds, but planets are too massive to be affected. The idea of using this wind with a solar sail has been proposed for interplanetary travel. The deep vacuum of space could make it an attractive environment for certain processes, for instance those that require ultraclean surfaces. In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720)

The quantum-mechanical vacuum

Even an ideal vacuum, thought of as the complete absence of anything, will not in practice remain empty. One reason is that the walls of a vacuum chamber emit light in the form of black-body radiation: visible light if they are at a temperature of thousands of degrees, infrared light if they are cooler. If this soup of photons is in thermodynamic equilibrium with the walls, it can be said to have a particular temperature, as well as a pressure. More fundamentally, quantum mechanics predicts that vacuum energy can never be exactly zero. The lowest possible energy state is called the zero-point energy and consists of a seething mass of virtual particles that have brief existence. This is called vacuum fluctuation. While most agree that this represents a significant part of particle physics, it is a concept that would benefit from a deeper understanding than currently available. Vacuum fluctuations may also be related to the so-called cosmological constant in the theory of gravitation, if indeed this entity were to be observed in nature on a macroscopic scale. The best support for vacuum fluctuations is the Casimir effect. In quantum field theory and string theory, the term "vacuum" is used to represent the ground state in the Hilbert space, that is, the state with the lowest possible energy. In free (non-interacting) quantum field theories, this state is analogous to the ground state of a quantum harmonic oscillator. If the theory is obtained by quantization of a classical theory, each stationary point of the energy in the configuration space gives rise to a single vacuum. String theory is believed to be analogous to quantum field theory but one with a huge number of vacua - with the so-called anthropic landscape.

Historical interpretation

Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers did not like to admit the existence of a vacuum, asking themselves "how can 'nothing' be something?". Plato found the idea of a vacuum inconceivable. He believed that all physical things were instantiations of an abstract Platonic ideal, and could not imagine an "ideal" form of a vacuum. Similarly, Aristotle considered the creation of a vacuum impossible—nothing could not be something. Later Greek philosophers thought that a vacuum could exist outside the cosmos, but not inside it. In the Middle Ages, the idea of a vacuum was thought to be immoral or even heretical. The absence of anything implied the absence of God, and hearkened back to the void prior to the story of creation in the book of Genesis. Medieval thought experiments into the idea of a vacuum considered whether a vacuum was present, if only for an instant, between two flat plates when they were rapidly separated. There was much discussion of whether the air moved in quickly enough as the plates were separated, or, following William Burley whether a 'celestial agent' prevented the vacuum arising—that is, whether nature abhorred a vacuum. This speculation became irrelevant after the Paris condemnations of Bishop Tempier, which required there to be no restrictions on the powers of God, which led to the conclusion that God could create a vacuum if he so wished. Following work by Galileo, Evangelista Torricelli argued in 1643 that there was a vacuum at the top of a mercury barometer. Some people believe that although Torricelli produced the first vacuum, it was Blaise Pascal who recognized it for what it was. Robert Boyle later conducted experiments on the effects of a vacuum. For example, a canary exposed to vacuum would rupture open due to the lack of pressure. In 1654, Otto von Guericke conducted his famous Magdeburg hemispheres experiment, showing that teams of horses could not separate two hemispheres from which the air had been evacuated. Concurrently, theories of the nature of light had proposed the idea of a aethereal medium which would be the medium to convey waves of light (Newton relied on this idea to explain refraction and radiated heat). This evolved into the luminiferous aether idea of the 19th century, but it was known to have significant shortcomings. In 1887 the Michelson-Morley experiment, using an interferometer to attempt to detect the change in the speed of light caused by the Earth moving with respect to the aether, was a famous null result, showing that there really was no static, pervasive medium throughout space and through which the Earth moved as though through a wind. (Of course, if the aether were the medium in which light waves traveled and electromagnetic and gravitational fields manifest, then it would be exceedingly difficult to distinguish the characteristics of such medium from those of the field or fields one was in. It would no more be possible to show that the Earth moved in relation to such an aether than it would be to illustrate that it moved in relation to its own electromagnetic and gravitational fields.)

External links

- [http://www.avs.org/ American Vacuum Society]
- [http://scitation.aip.org/jvsta/ Journal of Vacuum Science and Technology A]
- [http://scitation.aip.org/jvstb/ Journal of Vacuum Science and Technology B]
- [http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html Discussion of the effects on humans of exposure to hard vacuums].
- [http://www.arXiv.org/abs/hep-th/0012062 Vacuum Energy in High Energy Physics]
- [http://vacuumscientists.com/ Scientist of vacuum]
- http://www.mcallister.com/vacuum.html (Short History of Vacuum Terminology and Technology) Category:Industrial processes ja:真空

Crest

- Crest (heraldry) is a heraldic term.
- Crest (Physics) is the section of a wave that rises above an undisturbed position.
- Crest, Drôme is a commune of the Drôme département in France.
- Crest toothpaste is the name of a popular toothpaste in Canada, the USA, and the United Kingdom.
- CREST is a securities depository and settlement service of the United Kingdom and Ireland, operated by CRESTCo Ltd.
- CREST Newcastle is the Centre for Rehabilitation Engineering Studies.
- CREST syndrome is a limited form of the disease scleroderma.
- Crest of a Knave is an album by Jethro Tull
- The Crest Theatre is a historic theatre located in downtown Sacramento, California.
- A Sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and primate skulls.

Surfing

:See World Wide Web for "surfing" the web; see also Windsurfing Windsurfing Surfing (Hawaiian: he‘e nalu, "wave-sliding") is a very popular recreational activity and sport in which individuals are propelled across the water by the force of waves, while standing on a flat, wide board. Most modern surfboards are made of foam and fiberglass with one or more wooden strips or "stringers." An emerging surf technology is an epoxy surfboard, made from a different material. Epoxy boards are stronger and lighter than traditional fiberglass boards.

History

Originally developed by Hawaiian islanders (see Ngaru), before the 15th century, "he'e nalu" spread in the early 20th century to the mainland USA and Australia, where heavy timber "plank" boards were ridden directly towards beaches. The sport exploded in popularity in the 1950s and 1960s, when cheaper, more maneuverable, and lighter boards made of fiberglass and foam became available and the teenaged baby boomers headed to the beach in droves to enjoy the maneuverability and stunts made possible by the new boards. The sport has spread to most places where waves of sufficient size and shape appear, including France, Brazil, Costa Rica, Ireland, South Africa, Norway, and many island states. Equipment used in surfing includes a leash (to keep a surfer's board from washing to shore after a 'wipeout', and to prevent it from hitting other surfers), surf wax and/or traction pads (to keep a surfers feet from slipping off the deck of the board), and "skegs" (also known as fins) which can either be permanently attached ("glassed-on") or interchangable. In warmer climates swimsuits, surf trunks or boardshorts are worn; in cold water surfers can opt to wear wetsuits, booties, hoods, and gloves to protect them against lower water temperatures. Surfing's appeal probably derives from an unusual confluence of elements: adrenaline, skill, and high paced maneuvering are set against a naturally unpredictable backdrop—an organic environment that is, by turns, graceful and serene, violent and formidable. Surfers' skills are tested not only in their ability to control their board in challenging conditions, but by their ability to execute various maneuvers such as the 'cutback' (turning back toward the breaking part of the wave), the 'floater' (riding on the top of the breaking curl of the wave), 'off the lip' (banking off the top of the wave), the 'aerial' (arcing through the air above the wave) and, if the surf conditions allow it, tuberiding. This is the holy grail of surfing, where the surfer maneuvers into a position where the wave curls over the top of them, forming a "tube" (or "barrel"), with the rider inside the cylindrical portion of the wave. However, such situations do not exist if the waves 'dump' or 'close-out', meaning that they break in large parts at a time. The drama of surfing obscures the sport's mundane aspects. Most people only see the pros riding and miss (when televised) or ignore the time-consuming paddling out and waiting required to get a surfer into position. Competitive surfing is a comparison sport. Riders, competing in pairs or small groups, are allocated a certain amount of time to ride waves and display their prowess and mastery of the craft. Competitors are then judged according to how competently the wave is ridden, including the level of difficulty, as well as frequency, of maneuvers. There is a professional surfing world championship series held annually at surf beaches around the world. Although competitive surfing has become an extremely popular and lucrative activity, both for its participants and its sponsors, the sport does not have its origins as a competitive pursuit. It is common to hear debate rage between purists of the sport, who still maintain the ideal of 'soul surfing', and surfers who engage in the competitive and, consequently, commercial side of the activity. A non-competitive adventure activity involving riding the biggest waves possible (known as "rhino hunting") is also popular with some surfers. A practice popularised in the 1990s has seen big wave surfing revolutionised, as surfers use jetskis to tow them out to a position where they can catch previously unrideable waves (See also: tow-in surfing). These waves were previously unrideable due to the speed at which they travel. Some waves reach speeds of over 60 km/h; jetskis enable surfers to reach the speed of the wave thereby making them rideable. Jetskis not only allow surfers to ride these waves but allow them to survive 'wipeouts'. In many instances surfers would not survive the battering of the 'sets' (groups of waves together) without drowning. This spectacular activity is extremely popular with television crews, but because such waves rarely occur in heavily populated regions, and usually only a very long way out to sea on outer reefs, few spectators see such events directly. tow-in surfing catches a wave.]]

Understanding waves

Surfing conditions at a particular location or "break" that is known for surfing (see below) are almost never ideal. Wind blown consistently over a large area of fetch, or open water, generates waves. These waves use a drafting effect similar to race cars and cyclists to travel vast distances efficiently. To learn more about surf meteorology, see [http://www.stormsurf.com/page2/tutorials/menu.html StormSurf's Tutorials]. As waves near their ultimate destination (land), the bottom of the wave begins to run aground as the water becomes more shallow. There are two primary factors that contribute to the general characteristics of waves at a particular break: (1) the "swell window" or the exposure of the location to wave-generating areas of fetch, and (2) the structure of the ocean floor (composition, shape). The swell window determines the potential of a break to receive waves. In general, the western coast of any continent usually has better breaks since winds (and, therefore, waves) tend to travel from west to east. Coastlines that face east or south (in the Northern Hemisphere) or north (in the Southern Hemisphere) that are exposed to tropical storms and hurricanes can also be surfable on a consistent basis. When waves break along a section of coastline at an angle almost perpendicular to the land, these special locations, known as point breaks, can produce very long-lasting waves that can be surfed for several hundred meters. The two main types of waves for surfing apart from the pointbreak are the reef break (waves breaking over a coral reef or rockbed) and the beach break (waves breaking onto sand bars). To learn more about the types of waves for surfing see [http://www.surfing-waves.com/peeling_waves.htm Surfing-Waves.com Wave information]. The structure of the ocean floor is the biggest factor that determines the broad characteristics of waves at a particular break. For instance, there are beach breaks (soft sand bottom) that generate slower, mushy waves and reef breaks (coral reef or rock bottom) that tend to generate faster, more powerful waves. Based on the structure of the ocean floor, a location may break better on a particular tide, say, an incoming high tide or a low-low tide. Local wind conditions, water temperature, solar radiation, the crowd factor, hazardous aquatic life, water pollution, and aggression of local surfers are other factors that can have impact on the experience one might have surfing at a particular break. The availability of free model data from the NOAA has allowed the creation of several Surf forecasting websites. These automatically combine the above variables into a presentation of how good the surf will be.

Popular surfing areas

right Surfing is a global sport; one can find a surfer in almost every coastal nation in the world.
- France, particularly the Atlantic coast south of the Gironde
- Australia
- Newcastle, where Surfest is held annually.
- Gold Coast, Snapper Rocks and Burleigh Heads where many surf comps are held anually
- Ocean beaches of Sydney, in particular Bondi Beach, North Narabeen and Dee Why
- Victorian beaches Jan Juc and Bells Beach where the annual Rip Curl Pro is held every year.
- Western Australia beaches Margaret River
- The Atlantic coast of France (eg. Biarritz)
- Brazil
- Peru
- Cabo Blanco
- Pico Alto (home to the Mavericks of South America)
- Cerro Azul
- San Gallan
- Chicama (home of the longest left in the world)
- Mexico
- Baja States of Baja California Norte and Baja California Sur; Several great breaks, the island of Todos Santos being the most famous.
- Mainland – States of Sinaloa, Jalisco, Colima (home to Boca de Pascuales and its massive beachbreak), Michoacán (where rural surf towns abound), Guerrero, Oaxaca (where Puerto Escondido, the "Mexican Pipeline", is located), and Chiapas.
- Gulf Coast
- Indonesia
- Ireland
- The Maharees - South West Ireland County Kerry
- Easkey North West coast near Sligo
- New Zealand
- Manu Bay and Whale Bay, Raglan
- Bay of Plenty and East Coast, Mount Maunganui
- Much of South Africa's coastline (just a few listed)
- Amanzimtoti
- Cape St. Francis (Seal Point)
- Durban
- Elands Bay
- Jeffreys Bay
- Mossel Bay
- Scottburgh
- Port Alfred
- Port Elizabeth
- United Kingdom
- Fistral Beach in Newquay, Cornwall
- Croyde Bay in North Devon
- The Gower Peninsula near Swansea, Wales
- United States
- Northern California, while it has its moments at times, is a far less consistent place to surf than Southern California, with consistently poorer wind and wave conditions. The main problem is that most spots are too exposed to wind and swell, often resulting in "stormy" surf. Nor Cal is home to one of the most revered and dangerous spots in the world, Mavericks, for which there is no comparison in So Cal.
- Southern California, from San Diego to above Santa Barbara, features outstanding beaches such as Windansea, Tourmaline Park, Ponto, Lunada Bay, Huntington Beach, San Onofre, and Rincon, and is where American surfing music and culture began to evolve. This stretch of coastline is remarkable for the sheer number of consistently pleasant and surfable breaks.
- Hawaii is probably the most famous surfing mecca that exists, every year thousands upon thousands of surfers make the trip to pay respect to the birthplace of surfing. The North Shore of Oahu is home to perhaps the best stretch of surfing waves in the world, including Sunset Beach, Waimea Bay, and the world's most renowned and revered wave, "Pipeline" (or "Banzai Pipeline"), so named for the yawning chasms it regularly hurls over the heads of awe-struck surfers. The North Shore is the epicenter of commercial surfing each fall as it hosts a series of contests that end the professional season at Pipeline.
- The eastern central coast of Florida, particularly Brevard County, is renowned as the "small wave surfing capital of the world," and is home to such surfing luminaries as Kelly Slater, Todd Holland, and Matt Kechele.
- The Mid-Atlantic region includes popular spots such as North Carolina's Outer Banks, Long Island, Virginia Beach, Ocean City (Md.), and the Jersey Shore.
- There are decent breaks all up the east coast, notably on Cape Cod
- Even areas along the Great Lakes get local windswells with fresh-water barrels. The west coast of the Americas tends to have better surfing areas than the east coast. While the continental shelf of the west coast drops off quickly, on the east it extends a great distance, creating drag and making smaller and less powerful waves.
- Puerto Rico
- The West Coast in the Island has A-frame breaks, with international surfers coming every season for the taste of huge waves. The North Coast has consisten overhead spectacular breaks prolonged months of the year. The Sourthern and Eastern part of the Island have good breaks that don't get ridden that often. Anywhere else waves hit the shore. Many surfers are seen as territorial, hence the expression "locals only"; or as the rock group The Surf Punks put it, "my beach, my wave, my girl, so f--- you!". Other surfers, however, known as "soul surfers", hold less aggressive views towards others. These surfers see surfing as more than a sport; it is an opportunity to harness the waves in and to relax and forget about their daily routines. This type of surfing has seen a rise in popularity recently.

Surfing culture

See surf culture

Surfing movies

- Gidget (1959)
- Ride the Wild Surf (1964)
- Beach Party (1963)
- Beach Blanket Bingo (1965)
- Endless Summer (1966)
- Five Summer Stories (1972)
- Big Wednesday (1978)
- Apocalypse Now (1979) (scene with Robert Duvall, "Charlie don't surf!")
- North Shore (1987)
- Point Break (1991)
- Endless Summer II (1994)
- In God's Hands (1998)
- Blue Crush (2002)
- Step Into Liquid (2003)
- Riding Giants (2004)

Surf brands

- Billabong
- Rip Curl
- Quiksilver
- Mambo

Famous and notable surfers

- Duke Paoa Kahanamoku, olympian and Ambassador of Surfing
- Laird Hamilton, Hawaii, California, Big wave Rider and tow-in surfing inventor
- Tom Blake, Early 20th century surf pioneer, added fins to surfboards
- Bob Simmons, Initiated change in surfboards from flat logs to modern styles
- Keala Kennelly, Kauai
- Greg Cipes, United States
- Greg Noll, big wave pioneer, rode biggest wave of his era at Makaha
- Mark Richards, Newcastle. Australia, dominant surfer of the twin fin era
- Scott Bass Surfer Magazine online editor and pioneer of stand-paddle surfing
- Rell Sunn, Queen of Makaha, O‘ahu
- Jake Mattocks, Mr. SP
- Terry "TubeSteak" Tracy, The Original Big Kahoona http://www.tubesteak.org
- Gerry Lopez, Mr. Pipeline
- Shaun Tomson, one of the last top pros of the single fin era
- Bill Andrews, La Jolla Local http://adaywithba.com
- George_Freeth
- Simon Anderson Australian, first to win competitions on thrusters
- Tom Curren, First dominant pro thruster surfer
- Kelly Slater, Florida, considered one of the best surf competitors ever
- Andy Irons, Kauai
- Sunny Garcia
- Shane Dorian
- Bethany Hamilton, Kauai and shark attack survivor
- Layne Beachley, Australia
- John Whitmore introduced surfboards to SA and pionered many advances in surfboards and techniques. Whitmore befriended filmmaker Bruce Brown and provided much assistance in the creation of Endless Summer 1 and 2.
- Bruce Gabreilson, founder of official high school surfing leagues and creator of Internet's first surfing site

External links

- [http://www.wannasurf.com/ wannasurf.com] A comprehensive atlas to over 5000 surf spots around the world.
- [http://www.globalsurfers.com/ globalsurfers.com] A commercial surf-travel site with an atlas of surf spots around the world.
- [http://www.swellfinder.com/waves/ Swellfinder.com] Swell, wind & weather forecasts, surfcams, tides, etc. for windsurfers
- [http://www.wetsand.com Wetsand.com] Swell forecasts
- [http://www.magicseaweed.com magicseaweed.com] Swell and weather forecasts for breaks around the world Category:Surfing Category:Extreme sports Category:Water sports Category:Subcultures ja:サーフィン

Tsunami

on December 26, 2004.]] 2004 on December 2004]] A tsunami (pronounced soo-nah-mee or tsoo-nah-mee [ IPA or ]) is a series of waves generated when water in a lake or the sea is rapidly displaced on a massive scale. Earthquakes, landslides, volcanic eruptions and large meteorite impacts all have the potential to generate a tsunami. The effects of a tsunami can range from unnoticeable to devastating. The term tsunami comes from the Japanese language meaning harbour ("tsu", 津) and wave ("nami", 波 or 浪). Although in Japanese tsunami is used for both the singular and plural, in English tsunamis is well-established as the plural. The term was created by fishermen who returned to port to find the area surrounding the harbour devastated, although they had not been aware of any wave in the open water. A tsunami is not a sub-surface event in the deep ocean; it simply has a much smaller amplitude (wave heights) offshore, and a very long wavelength (often hundreds of kilometres long), which is why they generally pass unnoticed at sea, forming only a passing "hump" in the ocean. Tsunamis have been historically referred to as tidal waves because as they approach land they take on the characteristics of a violent onrushing tide rather than the sort of cresting waves that are formed by wind action upon the ocean (with which people are more familiar). However, since they are not actually related to tides the term is considered misleading and its usage is discouraged by oceanographers.

Causes

oceanographers A tsunami can be generated by any disturbance that rapidly displaces a large mass of water, such as an earthquake, volcanic eruption, landslide or meteorite impact. However, the most common cause is an undersea earthquake. An earthquake which is too small to create a tsunami by itself may trigger an undersea landslide quite capable of generating a tsunami. Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Such large vertical movements of the earth's crust can occur at plate boundaries. Subduction earthquakes are particularly effective in generating tsunamis, and occur where denser oceanic plates slip under continental plates in a process known as subduction. Sub-marine landslides; which are sometimes triggered by large earthquakes; as well as collapses of volcanic edifices, may also disturb the overlying water column as sediment and rocks slide downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can uplift the water column and form a tsunami. Waves are formed as the displaced water mass moves under the influence of gravity to regain its equilibrium and radiates across the ocean like ripples on a pond. In the 1950s it was discovered that larger tsunamis than previously believed possible could be caused by landslides, explosive volcanic action and impact events. These phenomena rapidly displace large volumes of water, as energy from falling debris or expansion is transferred to the water into which the debris falls. Tsunamis caused by these mechanisms, unlike the ocean-wide tsunamis caused by some earthquakes, generally dissipate quickly and rarely affect coastlines distant from the source due to the small area of sea affected. These events can give rise to much larger local shock waves (solitons), such as the landslide at the head of Lituya Bay which produced a water wave estimated at 50 – 150 m and reached 524 m up local mountains. However, an extremely large landslide could generate a megatsunami that might have ocean-wide impacts.

Characteristics

megatsunami). In fact, a tsunami is better understood as a new and suddenly higher sea level, which manifests as a shelf or shelves of water. The leading edge of a tsunami superficially resembles a breaking wave but behaves differently: the rapid rise in sea level, combined with the weight and pressure of the ocean behind it, has far greater force.]] Although often referred to as "tidal waves", a tsunami does not look like the popular impression of "a normal wave only much bigger". Instead it looks rather like an endlessly onrushing tide which forces its way around and through any obstacle. Most of the damage is caused by the huge mass of water behind the initial wave front, as the height of the sea keeps rising fast and floods powerfully into the coastal area. The sheer weight of water is enough to pulverise objects in its path, often reducing buildings to their foundations and scouring exposed ground to the bedrock. Large objects such as ships and boulders can be carried several miles inland before the tsunami subsides. Tsunamis act very differently from typical surf swells; they are phenomena which move the entire depth of the ocean (often several kilometres deep) rather than just the surface, so they contain immense energy, propagate at high speeds and can travel great trans-oceanic distances with little overall energy loss. A tsunami can cause damage thousands of kilometres from its origin, so there may be several hours between its creation and its impact on a coast, arriving long after the seismic wave generated by the originating event arrives. Although the total or overall loss of energy is small, the total energy is spread over a larger and larger circumference as the wave travels, so the energy per linear meter in the wave decreases as the inverse power of the distance from the source. This is the two-dimensional equivalent of the inverse square law in three dimensions. A single tsunami event may involve a series of waves of varying heights; the set of waves is called a train. In open water, tsunamis have extremely long periods (the time for the next wave top to pass a point after the previous one), from minutes to hours, and long wavelengths of up to several hundred kilometres. This is very different from typical wind-generated swells on the ocean, which might have a period of about 10 seconds and a wavelength of 150 metres. The actual height of a tsunami wave in open water is often less than one metre. This is often practically unnoticeable to people on ships. The energy of a tsunami passes through the entire water column to the sea bed, unlike surface waves, which typically reach only down to a depth of 10 m or so. The wave travels across the ocean at speeds from 500 to 1,000 km/h. As the wave approaches land, the sea shallows and the wave no longer travels as quickly, so it begins to 'pile-up'; the wave-front becomes steeper and taller, and there is less distance between crests. While a person at the surface of deep water would probably not even notice the tsunami, the wave can increase to a height of 30 m or more as it approaches the coastline and compresses. The steepening process is analogous to the cracking of a tapered whip. As a wave goes down the whip from handle to tip, the same energy is deposited in less and less material, which then moves more violently as it receives this energy. A wave becomes a 'shallow-water wave' when the ratio between the water depth and its wavelength gets very small, and since a tsunami has an extremely large wavelength (hundreds of kilometres), tsunamis act as a shallow-water wave even in deep oceanic water. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s²) and the water depth. For example, in the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s (720 km/h or 450 mi/h) with little energy loss, even over long distances. At a water depth of 40 m, the speed would be 20 m/s (about 72 km/h or 45 mi/h), which is much slower than the speed in the open ocean but the wave would still be difficult to outrun. Tsunamis propagate outward from their source, so coasts in the "shadow" of affected land masses are usually fairly safe. However, tsunami waves can diffract around land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India). They also need not be symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography. Local geographic peculiarities can lead to seiche or standing waves forming, which can amplify the onshore damage. For instance, the tsunami that hit Hawaii on April 1, 1946 had a fifteen-minute interval between wave fronts. The natural resonant period of Hilo Bay is about thirty minutes. That meant that every second wave was in phase with the motion of Hilo Bay, creating a seiche in the bay. As a result, Hilo suffered worse damage than any other place in Hawaii, with the tsunami/seiche reaching a height of 14 m and killing 159 inhabitants.

Signs of an approaching tsunami

The following have at various times been associated with a tsunami [http://www.pmel.noaa.gov/tsunami/PNG/Upng/Davies020411/]:
- An earthquake may be felt.
- Large quantities of gas may bubble to the water surface and make the sea look as if it is boiling.
- The water in the waves may be unusually hot.
- The water may smell of rotten eggs (Hydrogen Sulphide) or of petrol or oil.
- The water may sting the skin.
- A thunderous boom may be heard followed by
- a roaring noise as of a jet plane
- or a noise akin to the perodic whoop-whoop of a helicopter,
- or a whistling sound.
- The sea may recede to a considerable distance.
- A flash of red light might be seen near the horizon.
- As the wave approaches the top of the wave may glow red.

Warnings and prevention

Tsunamis cannot be prevented or precisely predicted, but there are some warning signs of an impending tsunami, and there are many systems being developed and in use to reduce the damage from tsunamis. In instances where the leading edge of the tsunami wave is its trough, the sea will recede from the coast half of the wave's period before the wave's arrival. If the slope is shallow, this recession can exceed many hundreds of metres. People unaware of the danger may remain at the shore due to curiosity, or for collecting fish from the exposed sea bed. Hilo, a tsunami struck Kamakura, destroying the wooden building that housed the colossal statue of Amida Buddha at Kotokuin. Since that time, the statue has been outdoors.]] In instances where the leading edge of the tsunami is its first peak, succeeding waves can lead to further flooding. Again, being educated about a tsunami is important, to realise that when the water level drops the first time, the danger is not yet over. In a low-lying coastal area, a strong earthquake is a major warning sign that a tsunami may be produced. Regions with a high risk of tsunamis may use tsunami warning systems to detect tsunamis and warn the general population before the wave reaches land. In some communities on the west coast of the United States, which is prone to Pacific Ocean tsunamis, warning signs advise people where to run in the event of an incoming tsunami. Computer models can roughly predict tsunami arrival and impact based on information about the event that triggered it and the shape of the seafloor (bathymetry) and coastal land (to