2.1.2 Refraction

When light passes from one transparent medium to another of different density, its speed changes. This effect is known as refraction. It is the reason that a straight stick emerging from water appears bent at the surface of the water, and why water viewed from above appears shallower than it really is. The change in the speed of light between air and the medium, expressed as a ratio, is the refractive index of the medium. (Note that this is a simplification. Strictly, refractive index is defined as the ratio of the speed of light in a vacuum to its speed in the medium. However, the speed of light in air is very close to that in a vacuum.)