Refraction
Refraction, the bending of waves, such as light waves, when they pass from one substance to another. Refraction occurs because waves travel at different speeds through different substances.
In the illustration The Cause of Refraction, arrows represent the direction and speed of light waves in a beam of light. The speed of light is less in the glass than in the air. The lines connecting the arrow tips represent wave fronts (parts of the waves in phase with each other). Upon entering the glass, a wave front bends because one part of it is moving at a slower speed than the other. The arrows also bend because the direction in which the light advances is always perpendicular to the wave front. Once within the glass, the entire wave front travels at the same speed and no bending occurs. In leaving the glass, different parts of the front again travel at different speeds and refraction again takes place.
The different colors of light are refracted by different amounts. Hence, light passing through a prism (under proper conditions) is broken up into the colors of the rainbow. Refraction is responsible for mirages, rainbows, the twinkling of stars, the apparent bending of objects protruding from water, and the colors of sunrise and sunset. Refraction by lenses is used to focus light.
The index of refraction of a substance is the ratio of the speed of light in a vacuum to its speed in the substance. The higher the figure, the greater is the amount of refraction. This figure varies from substance to substance, but for a given substance is always the same. This fact allows scientists and technicians to identify substances and determine their purity by measuring their index of refraction. A real diamond, for example, has an index of nearly 2.5; that of imitation diamonds rarely exceeds 2.