REFRACTION

Early humans doubtless noticed refraction as well as reflection; a clear pond looks shallower than it actually is because of this effect. Refraction is associated with the fact that different media transmit light at different speeds. This does not violate the fundamental principle of relativity theory. The speed of light is absolute in a vacuum, where it travels at about 299,792 km/s (186,282 mi/s), but light travels more slowly than this in other media.

In air, the difference in the speed of light is slight compared with its speed in a vacuum, although it can be significant enough to produce refractive effects at near-grazing angles between air masses having different densities. In water, glass, quartz, diamond, and other transparent media, light travels quite a lot more slowly than it does in a vacuum. The refractive index, also called the index of refraction, of a medium is the ratio of the speed of light in a vacuum to the speed of light in that medium. If c is the speed of light in a vacuum and c_m is the speed of light in medium M, then the index of refraction for medium M, call it r_m, can be calculated simply:

r_m = c/c_m
Always use the same units when expressing c and c_m. According to this definition, the index of refraction of any transparent material is always greater than or equal to 1.
The greater the index of refraction for a transparent substance, the more light is bent when it passes the boundary between that substance and air. Different types of glass have different refractive indices. Quartz refracts more than glass, and diamond refracts more than quartz. The high refractive index of diamond is responsible for the multicolored shine of diamond stones.