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The Refractive index, also called index of refraction, measure of the bending of a ray of light when passing from one medium into another. If i is the angle of incidence of a ray in vacuum (angle between the incoming ray and the perpendicular to the surface of a medium, called the normal) and r is the angle of refraction (angle between the ray in the medium and the normal), the refractive index n is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction; i.e., n = sin i / sin r. Refractive index is also equal to the velocity of light c of a given wavelength in empty space divided by its velocity v in a substance, or n = c/v.

The bending of a light ray as it passes from air to water is determined by Snell's law. This law also applies to the bending of light by lenses and to the guiding of light by the fiber optic cables that carry modern communications signals. In part 1 of this lab you will investigate the bending of light as it travels from air to plastic and plastic to air. Your data will illustrate Snell's law. In part 2, you will find that the speed of light in plastic varies slightly with the wavelength.

The speed of light in a vacuum is c = 3.00 x 10⁸ m/s. Because of a complex interaction between an electromagnetic wave (light) and the charges in matter(electrons and protons), a light signal travels more slowly in transparent materials, such as glass or water, than in vacuum. The ratio of c, the speed of light in a vacuum, to v, the speed of light in a medium, is called the index of refraction, n, of the medium

List of refractive indices for common materials

Material	Refractive Index
Air	1.0003
Water	1.333
Glycerin	1.473
Immersion Oil	1.515
Glass (Crown – convex lens)	1.520
Glass (Flint – concave lens)	1.656
Zircon	1.920
Diamond	2.417
Lead Sulfide (galena)	3.910

This calculator calculates the index of refraction using velocity of light in vacuum, velocity of light in the medium values.