Refraction
When a beam of light enters a medium in which its velocity is different from that in its
original medium, the path of the beam changes. This bending of the beam, called
refraction, is characteristic of all waves--sound, radio, and mechanical. Most experience
with refraction, however, is with light. The principles of refraction are used in a
variety of optical devices: eyeglasses, microscopes, cameras, and binoculars.
Snell's Law
The basic law of refraction, first proposed in 1621 by the Dutch scientist Willebrord
Snell, relates the magnitude of refraction to the velocity of light in the two media
(equivalent to the index of refraction). If i is the angle the incident beam makes with
the normal (in optics, all such angles are measured from the normal, a line perpendicular
to the surface), and r the angle of the refracted beam, then
When light entering a denser medium, from air into glass,
for example, is bent towards the normal; on entering a less dense
medium, light is bent away from the normal.
If light goes through a substance with parallel
surfaces, such as a window, the beam that emerges will be parallel to its
incident beam but laterally displaced, or shifted.
