Refractive index

The refractive index or index of refraction is a ratio of the speed of light in a vacuum relative to that speed through a given medium (this quantity does not refer to an angle of refraction, which can be derived from the refractive index using Snell's Law). ^{[note 1]} In other words, as light passes from one medium to another as from air to water, the result is a bending of light rays at an angle. This physical property occurs because there is a change in the velocity of light going from one medium into another. Refractive index also describes the quantity that light is bent as it passes through a single substance. This involves calculating the angle at which light enters the medium and comparing that with the angle at which the light leaves the medium.^[1]^[2]^[3]

Another view rates each substance with its own refractive index. This is because the velocity of light through the substance is compared as a ratio to the velocity of light in a vacuum. The velocity at which light travels in a vacuum is a physical constant, and the fastest speed at which energy or information can travel. However, light travels slower through any given material, or medium, that is not a vacuum. This is actually a delay from when light enters the material to when it leaves; i.e., when some is absorbed, and another part transmitted (See: light in a medium).^[1]^[3]^[4]^[5]

A simplified, mathematical description of refractive index is as follows:

RI = velocity of light in a vacuum / velocity of light in medium

Hence, the RI of water is 1.33, meaning that light travels 1.33 times faster in a vacuum than it does in water.

Refractive index is also frequency dependent. In other words, the refractive index will vary according to the frequency of radiated light. This results in a slightly different refractive index for each color. Measurements are normally taken using the yellow light of a sodium source. Therefore, the cited values of refractive indexes such as 1.33 for water, are based on yellow light at a wavelength of 589.3 nanometers. One final note: temperature also affects refractive index, and cited values are based on a standard temperature.^[1]^[2]^[3]

http://en.wikipedia.org/wiki/Refractive_index.

SHERLYN.