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Jai Ganesh

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Registered: 2005-06-28

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Refraction

Refraction

Gist

Refraction is the bending of a wave—most commonly light—as it passes obliquely from one transparent medium to another, caused by a change in its speed. It occurs because light travels at different velocities in materials of different densities (e.g., passing from air into water). This phenomenon is essential for lenses, prisms, and human vision.

Refraction is the bending of a wave, like light, sound, or water waves, as it passes from one transparent medium (like air) to another (like water or glass) due to a change in its speed, causing a change in direction. This common phenomenon explains why a straw in a glass of water looks bent, and it's essential for lenses, glasses, cameras, and even the formation of rainbows. 

Summary

Refraction, in physics, is the change in direction of a wave passing from one medium to another caused by its change in speed. For example, waves travel faster in deep water than in shallow. If an ocean wave approaches a beach obliquely, the part of the wave farther from the beach will move faster than the part closer in, and so the wave will swing around until it moves in a direction perpendicular to the shoreline. The speed of sound waves is greater in warm air than in cold. At night, air is cooled at the surface of a lake, and any sound that travels upward is refracted down by the higher layers of air that still remain warm. Thus, sounds, such as voices and music, can be heard much farther across water at night than in the daytime.

The electromagnetic waves constituting light are refracted when crossing the boundary from one transparent medium to another because of their change in speed. A straight stick appears bent when partly immersed in water and viewed at an angle to the surface other than 90°. A ray of light of one wavelength, or colour (different wavelengths appear as different colours to the human eye), in passing from air to glass is refracted, or bent, by an amount that depends on its speed in air and glass, the two speeds depending on the wavelength. A ray of sunlight is composed of many wavelengths that in combination appear to be colourless. Upon entering a glass prism, the different refractions of the various wavelengths spread them apart as in a rainbow.

Details

Refraction is the bending of a wave when it enters a medium where its speed is different. The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell's Law.

The bending of refraction can be visualized in terms of Huygens' principle. As the speed of light is reduced in the slower medium, the wavelength is shortened proportionately. The frequency is unchanged; it is a characteristic of the source of the light and unaffected by medium changes.

The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium.

Snell's Law relates the indices of refraction n of the two media to the directions of propagation in terms of the angles to the normal. Snell's law can be derived from Fermat's Principle or from the Fresnel Equations.

If the incident medium has the larger index of refraction, then the angle with the normal is increased by refraction. The larger index medium is commonly called the "internal" medium, since air with n=1 is usually the surrounding or "external" medium. You can calculate the condition for total internal reflection by setting the refracted angle = 90° and calculating the incident angle. Since you can't refract the light by more than 90°, all of it will reflect for angles of incidence greater than the angle which gives refraction at 90°.

Additional Information

In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction. How much a wave is refracted is determined by the change in wave speed and the initial direction of wave propagation relative to the direction of change in speed.

Optical prisms and lenses use refraction to redirect light, as does the human eye. The refractive index of materials varies with the wavelength of light, and thus the angle of the refraction also varies correspondingly. This is called dispersion and allows prisms and raindrops in rainbows to divide white light into its constituent spectral colors.

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