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Seeing Dark Lines

In the Bubble Colors activity, we saw how light waves could cancel each other to produce the colors we see in a soap bubble. In that experiment, we only canceled out part of the light, removing some colors so we could see others. This time, we are going to cancel it entirely to let you see lines of darkness while you are looking at a light.

You will need:

  • your fingers
  • a bright light

Sit a few feet away from the light. Hold your hand up in front of your face, with your fingers held out straight. If you look, you will see that there are open gaps between your fingers. Start with your hand about an inch away from your eye, looking at the light through one of these gaps. Slowly move your hand away from your face. You should begin to see dark stripes or bands inside the gap. If you have trouble seeing them, squeeze your fingers closer together or move them a little apart, until they are easy to see.

These lines are called diffraction fringe. What causes them? As the light moves through the slit between your fingers, it bends and begins to spread outwards. This is called diffraction. As the light spreads, the light waves meet each other and something wonderful happens.

Picture waves in water as they meet each other. Each wave has a part that is up (the crest) and a part that is down (the trough). If both waves are up when they come together, they add their heights and you get a very high crest as they meet. If both are down, you get a very deep trough. But what happens if one is up and the other is down? They cancel each other out and you get flat water. In this case, instead of flat water, you get dark areas.

This experiment is based on the work of Thomas Young. He was an English scientist who in 1820 used a similar experiment in his study of light. Light is a strange thing. In some ways, it acts as if it is made of tiny particles. In other ways, it acts as if it is made up of waves. This experiment is an example of light acting as waves. If two particles hit each other, they don't both vanish. The only way that Thomas Young could think of that two beams of light could meet and cancel each other out was if light was made up of waves.

In Young's time there were bitter arguments between scientists, with some saying that light was made or particles and others saying that light was made of waves. Now we take a much more practical approach. We say that light is made of photons, which can act like particles in some ways and like waves in others. That leaves more time to argue about other puzzles in science.