What is a prism and where do those colors come from?

The first sentence of the Wikipedia article reads as follows: “In optics, a prism is a transparent optical element with flat, polished surfaces that refract light.”

Which means: A prism is a chunk of glass that forms a rainbow of colors when light shines through it.

That’s better.

But why does it do that?

Let us return to Isaac Newton, the man with the apple who discovered gravity.

Before Newton, folks thought that light itself was colorless, and the glass composing the prism was responsible for the colors in the exiting rays. Not so, the young Newton surmised. He devised an experiment. Newton passed red colored light from one prism through a second prism and found the light was still red. From this, he theorized the color must be present in the light itself and not created by the prism.

To prove that the color was in the light and not the glass, our budding professor devised another experiment.

The thoughtful Newton took two prisms. He passed white light through the first and got the pretty red, yellow, green, cyan, blue and magenta rainbow we all enjoy seeing on the kitchen table from the crystal in the china cabinet or on the living room walls from that very nice diamond wedding ring. Well, our smart I.N. caught that little rainbow in a lens and bent it back at the second prism. Into the prism those pretty colors went and out they came all white! What? No colors whatsoever. Newton has recomposed the light back to its original white color.

The prism does not create the colors. The prism separates the colors that are already there. Light with all its colors looks white to us. But when white light moves through a glass prism, the internal crystalline structure of the glass or diamond slows down some colors more than other. (Think of runners jumping over hurdles – one runner is shorter and gets slowed down by the hurdles more than a tall runner who reaches the finish line first, even though both runners are as fast on a straight track.) On the other side of the prism, we see the taller colors first, followed by the shorter ones. That’s the rainbow we see – the order of the colors at the finish line depending on how the hurdles in the prism slowed down and separated the runners.

Different colors are impeded more or less as they traverse a prism and appear on the other side in the order of a rainbow.

Isaac was a smartie. He figured in his head that if he focused all those rainbow colors back on another prism, he bet the shorter ones would get through the prism at the same time as the taller colors. And he was right! The short guys and tall guys all finished together in a blur of white. Hurray!! Cool race. Do it again. That was neat.

And it was, because light is composed of different colors – some shorter and some taller. When they move through a glass prism, some are slowed more than others, and the light on the other side is a rainbow of spread-out finishers – who eventually all catch up with each other, and there’s that white light again. But that rainbow sure was fun.

Keep thinking. Isaac did. He figured it out.

And so can you.

I bet,

Grandpa Jim