In optics the refractive index or index of refraction n of a substance (optical medium) is a dimensionless number that describes how light, or any other radiation, propagates through that medium. It is defined as
where c is the speed of light in vacuum and v is the speed of light in the substance. For example, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in vacuum as it does in water. (See typical values for different materials here.)
The historically first occurrence of the refractive index was in Snell's law of refraction, n1sinθ1= n2sinθ2, where θ1 and θ2 are the angles of incidence of a ray crossing the interface between two media with refractive indices n1 and n2.
Brewster's angle, the critical angle for total internal reflection, and the reflectivity of a surface also depend on the refractive index, as described by the Fresnel equations.
The refractive index can be seen as the factor by which the velocity and the wavelength of the radiation are reduced with respect to their vacuum values: The speed of light in a medium is [pic] and similarly the wavelength in that medium is [pic], where [pic]is the wavelength of that light in vacuum. This implies that vacuum has a refractive index of 1. Historically other reference media (e.g.,air at a standardized pressure and temperature) have been common.
Refractive index of materials varies with the wavelength. This is called dispersion; it causes the splitting of white light in prisms andrainbows, and chromatic aberration in lenses. In opaque media, the refractive index is a complex number: while the real part describes refraction, the imaginary part accounts for absorption.
The concept of refractive index is widely used within the full electromagnetic spectrum, from x-rays to radio waves. It can also be used with wave phenomena other than light (e.g., sound). In this case the speed of sound is used instead...