Did you ever look at the edge of a mirror, where the glass is cut at an angle, and see all the colors of the rainbow there? The glass was acting as a prism. What was taking place was the “dispersion” of light; what you saw was the “spectrum.” Now let’s see what all of this means.

When light passes from air into water, or from air into glass, it is changed in direction. This change is called “refraction,” and you’ve probably noticed it thousands of times! For example, you’ve seen a spoon slanting into a glass of water. At the surface, the spoon looks as if there’s a sharp bend in its handle. This is because the water has bent the beam of light slightly.

Now let’s get back to our prism. When a beam of light strikes the glass at an angle, it is bent. Its speed actually becomes less. This is refraction. But instead of coming out as the white light that went in, it comes out in all the colors of the spectrum! Why is this? It’s because white light is not a special kind of light, it is a mixture of all colors.

So when white light enters the prism, all the colors in the light are being bent or refracted. But not all the colors are bent the same way! The red light is bent, or refracted, the least. So it appears at the top of the spectrum, or rainbow of colors. Next to it is the orange, then the yellow, green, blue, indigo, and at the other end, the violet, which is bent the most. And that’s why we see all the colors that are in white light coming out separately when light goes through a prism.

Man had known for a long time that this happened, but it was explained by saying that the water or glass changed the light in some way. Then Sir Isaac Newton, in 1666, performed an experiment that gave the correct explanation. Newton let sunlight into a darkened room through a narrow slit, placed a prism in the path of the light, and studied the spectrum thrown on the wall. He called the spreading effect “dispersion,” and he proved that white light is a mixture of all colors.