The extent to which a beam of light bends when it hits a different medium depends on the indices of refraction of the medium as well as the medium from which it came and the angle at which the light strikes the boundary between the two media.

All materials have an index of refraction that depends on the speed of light in the material. The index of refraction is the speed of light in a vacuum divided by the speed of light in the material. A vacuum has a refractive index of 1, water is 1.33, and glass is around 1.5. The higher the index of refraction, the more slowly light travels through the medium.

Snell’s Law of Refraction, named after Dutch physicist Willebrord Snellius (1580-1626), tells us how light behaves at a boundary between two different media. Consider the interface between two media where the refractive index of the top medium is lower than that of the bottom medium. According to Snell’s Law, when light hits the boundary between two materials it is bent from its original path to a smaller angle with respect to the line perpendicular to the surface of the interface. As the incoming, or incident angle increases, so does the refracted angle. When light goes from a medium with a higher refractive index into one with a lower index then it is bent away from the line perpendicular to the surface.