Refraction: Bending the Light

Light not only bounces off surfaces, it goes through some of them, often slowing down and changing direction in the process. This directional change, or "bending," is known as refraction, and it occurs at the point where light passes from one medium to another of different density. In the air, light travels at 186,000 miles per second; but water, which is denser than air, slows light down by about one fourth. Glass, which is denser yet, slows it down by a third, and diamond still more. However, for any sort of refraction to take place, the light must strike the new medium at an angle, not head on. The size of this angle determines the amount of bending, a phenomenon illustrated in the photograph above with transparent plastic blocks. Entering from the left, the three light beams hit the first block head on and pass through without bending. But they hit the next block at an angle, causing some of their light to be reflected upward. Most of it, however, enters the block where, slowed by the greater density of the plastic, the beams are bend downward--only to resume their original direction and speed as they leave the block. The third block's two concave surfaces spread the beams apart, but the last block acts as a convex lens and refracts them back together so sharply that they actually cross each other at the right.

The refraction of light produces mirages, rainbows and such bizarre optical effects as the distortion of the girl sitting by the pool at extreme right. It makes a thick-walled glass beer mug look fuller than it really is, and makes the sun appear to set several minutes later than it really does. It also makes it possible to remedy the often faulty refraction in the human eye with corrective eyeglasses.


Life Science Library - Light and Vision