How does a prism separate colors?

1. A beam of sunlight shines through a glass prism on the windowsill, and a band of colors appears on the wall. Where did the colors come from?
2. White light is really many colors travelling together. Red, orange, yellow, green, blue and violet all mixed up look white to your eyes.
3. Roll a cart at a slant onto carpet. One wheel slows first, so the cart turns. Light slows down in glass, so it turns in just the same way.
4. Here is the trick. Glass slows violet light a bit more than red light. So violet bends the most, red bends the least, and the colors fan out.
5. On a screen, the colors line up in order: red, orange, yellow, green, blue, indigo and violet. This band of colors is called a spectrum.
6. A rainbow in the sky is made the same way. Sunlight goes into each raindrop, bends, bounces off the back, and bends again on the way out, spread into colors.

A lamp sends a thin beam of white light through a prism onto a screen. Tap each part to see what happens there.

• Lamp with a slit. A lamp in a box with a narrow slit. Only a thin beam of white light gets out, so its path is easy to follow.
• White beam. The white beam. All the colors travel together along the same path, so you cannot tell them apart yet.
• Prism. A solid block of clear glass shaped like a triangle. Its slanted faces are what make it special.
• Way in. The beam enters the glass at a slant. It slows down and bends. The colors begin to part, but only a tiny bit.
• Way out. Leaving the glass, the light speeds up and bends again, the same way as before. The colors spread farther apart.
• Red light. Red light is slowed the least by glass, so it bends the least. It always lands at one end of the rainbow.
• Violet light. Violet light is slowed the most, so it bends the most. It always lands at the opposite end from red.
• Screen. A white card catches the colors. The farther it is from the prism, the wider the rainbow.

Turn the prism and watch the colors move. Then send just one color through and follow its path.

Experiment: Which color of light does a prism bend the most?

1. Turn the prism until all the colors land on the screen.
2. Pick the red light, then the violet light. Which one is turned farther from the straight path?
3. Go back to white light. Where on the screen do red and violet land?

What we found: Violet light bends the most and red the least, with the other colors in order in between. That is why a prism always makes the colors in the same order.

Turn on JavaScript to try it yourself.

Each lantern glows only in its own color. Turn the prism and slide the lantern rack until red, green and violet all glow. Then press Lock it in.

Sunlight and lamp light look white, but white light is really a mixture. Long ago, a scientist named Isaac Newton let a thin beam of sunlight into a dark room and through a glass prism. A band of colors appeared on the wall. He then used a second prism to squeeze the colors back together, and the light turned white again.

The colors separate because of bending. Light travels a little more slowly in glass than in air. When a beam hits glass at a slant, it turns, the way a cart turns when one wheel rolls onto carpet before the other. This bending is called refraction.

Here is the key: glass slows each color by a slightly different amount. Violet light is slowed the most, so it bends the most. Red is slowed the least, so it bends the least. Orange, yellow, green and blue fall in order between them. That is why the colors of a spectrum always come in the same order.

A flat window does not make rainbows, because the light bends one way going in and bends straight back coming out. A prism's faces are tilted, so both bends go the same way and the colors keep spreading. Raindrops work in a similar way: sunlight bends going into the drop, bounces off the back and bends again coming out, and millions of drops together paint a rainbow. The cart comparison has a limit, though: it cannot explain why colors differ. For that you need to think of light as waves.

Key words

Prism

A block of clear glass or plastic with flat faces that are tilted toward each other.

Refraction

The bending of light when it passes from one clear material into another.

Spectrum

The band of colors you get when white light is spread out.

Dispersion

The spreading of white light into its colors.

Refractive index

A number that tells how much a material slows light. A bigger number means more bending.

Deviation

The angle a ray has been turned through, compared with the straight path it started on.

Check your thinking

1. Where do the colors come from when white light passes through a prism?

   Show the answer

   They were already in the white light, mixed together. White light is a mixture of colors. The prism only spreads them out.

2. Which color of light is bent the most by a prism, and which the least?

   Show the answer

   Violet the most, red the least. Glass slows violet light the most, so it turns the most. Red is slowed the least.

3. Why doesn't a flat window pane make a rainbow like a prism does?

   Show the answer

   The light bends going in and bends straight back coming out. With flat, parallel sides the two bends cancel out. A prism's tilted sides bend the light the same way twice.

4. You move the screen farther away from the prism. What happens to the rainbow on it?

   Show the answer

   It gets wider. The colors leave the prism in slightly different directions, so the farther they travel, the farther apart they get.

5. What plays the part of the prism when a rainbow appears in the sky?

   Show the answer

   Millions of raindrops. Each raindrop bends the sunlight and spreads its colors, just like a tiny prism.