Refraction of a light ray on entering and leaving a glass block.
When light passes from one transparent substance to another, its speed changes. The bending which occurs when a beam of light crosses the boundary (between, for example, air and glass) obliquely, can be explained on the basis of the change in speed; one side of the beam is affected before the other. In the illustration, a beam passes through two glass blocks, first at right angles and then at an oblique angle.
a) The length of a chosen number of wavelengths of the light in the air. Due to the reduction in speed, the corresponding length in glass is a/1.5, = 2/3a; the figure 1.5 is the refractive index of the glass.
i) The angle at which the light beam reaches the surface of the glass.
r) The angle at which the beam enters the glass.
b) The distance which the right-hand side of the beam travels in air after the left-hand side of it has entered the glass. The corresponding distance in the glass, as before, is b/1.5. From the two outlined right angled triangles, it can be seen that (sin i) / (sin r) = the refractive index 1.5; this is the basic law of refraction.
Refraction is the bending of light from its original path as it passes from one transparent material to another with a different refractive index. This index is defined as equal to 1 in a vacuum. For any other medium it is defined as the speed of light in a vacuum divided by the speed of light in the medium. Thus refraction occurs when the speed of light is different in the two media. The amount of bending depends on the media involved and the frequency of the light.
The angle of refraction also depends on the light's frequency. Different frequencies refract at slightly different angles – a phenomenon known as dispersion. The most familiar example is the dispersion of white light into a spectrum of colors when it is passed through a prism.
The effect of refraction is used in the design of lenses and prisms, and of combinations of lenses such as eyepieces and refracting telescopes. In nature, refraction causes objects appear higher in the sky than they otherwise would. Objects more than halfway from the horizon to the zenith (i.e., with an altitude greater than 45°) are almost totally unaffected; however, objects near the horizon can be shifted by a degree or so. It is also responsible for the well-known phenomena of rainbows, mirages, and atmospheric haloes.