How Do Prisms Work

••• Photo courtesy of morgueFile
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In the 1600s, Isaac Newton did a series of experiments with prisms and light. He showed that prisms not only split light into the familiar rainbow colors, but can also recombine them. The glass of a prism, and the angles of its sides, work together to make a fascinating optical tool.

Effects of Light

When light passes from the air into glass, it slows down, and when it leaves the glass, it speeds up again. If the light hits the glass at an angle instead of dead-on, it undergoes refraction. The angle at which it hits the glass is not the same as the angle it travels inside the glass. The light is no longer moving in a straight line, but gets bent at the surface. The same thing happens when the light leaves the prism--it bends again.

Snell's Law

An optical principle called Snell’s Law predicts exactly how this happens. Snell’s Law deals with the angles that light enters and leaves a prism, and something called the index of refraction. The index of refraction shows how much light slows down when it goes into the glass.

Color Changes

The different colors of light, from red to violet, each get bent at slightly different angles. Red gets bent the least, violet the most. This causes the colors to fan out and become distinct.

Second Prism

The fact that a prism can break light into colors was known before Newton. But Newton asked what would happen if he put a second prism in the location of the colors. If the second prism caught all the colors on one of its surfaces, white light came out of the other side. The same properties that spread the colors apart worked in reverse to reassemble them.