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. If the i angle the incident beam makes with the normal, and the r angle of the refracted beam, then (sin i / sin r) = (index of refraction of the medium containing the refracted beam) / (index of refraction of the medium with the incident beam), thus, light entering a denser medium, from air into glass, for example, is bent toward 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.

A rainbow caused by
reflection once and refraction twice.

A secondary rainbow caused by
reflection twice and refraction twice.

rainbow

   Total Internal Reflection

    Suppose a beam of light passes from glass to air. Air has a lower index of refraction, so the emerging beam is bent away from the  normal. As the angle of incidence is increased, the beam in air moves farther from the normal until it makes a 90° angle with the normal and grazes the surface. The angle of incidence when this occurs is known as the critical angle. Increasing the angle incidence still further results in no light penetrating the glass-air boundary; instead, all the light is reflected back into the glass. This is known as total internal reflection, an effective way of reflecting light without using a mirror. One disadvantage of mirrors is that they have two reflecting surfaces, resulting in a faint "ghost," or double image. Total reflecting prisms are used in fine cameras, binoculars, and other optical instruments to change the direction of light. Light may be "transported" by transparent fibers and by glass or plastic rods; whether straight or curved, the light stays inside them because of successive internal reflections.

total internal reflection