In cases where light is not traveling through a vacuum, the speed at which it travels is not equal to c. This speed v is always less than c. The two quantities are related by
where n is known as the index of refraction. Refraction or bending of light occurs when light passes between two different media and is not reflected at the interface. The amount of bending is governed by Snell's Law:
Both angles are measured from the normal to the interface. If the speed of light in the second medium is slower than the speed in the first medium (i.e. is less than ), is smaller than and the light bends toward the normal.
