Changing the Speed of Light

This is a science demonstration that is often used as a magic trick, to make a coin suddenly appear. It is a very easy trick. All you have to do is change the speed of light.

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You will need:

  • a cereal bowl or some other shallow bowl that you cannot see through
  • a coin
  • water
  • a friend

Place the bowl on a table. Place the coin in the bottom of the bowl. When you stand beside the table, you should be able to see the coin. Now, back away from the table (carefully!) until you can no longer see the coin. If you make it all the way to the wall and can still see the coin, your bowl needs to be higher, so place it on top of some books or a box.

When you reach the point where you can no longer see the coin, stop. Stand still and have your helper pour some water slowly into the bowl. As the water level rises, you will suddenly be able to see the coin again!

What happened? You are seeing the results of refraction. As light moves from one substance to another with a different density, it changes its speed. That change in speed can cause the light to bend its path. That bending is called refraction.

You can see this bending by placing a pencil into a clear glass of water. Looking from the side, it will appear that the pencil is broken. The light that hits the pencil above the water line travels in a straight line, but the light that hits the pencil underwater is refracted or bent when it moves from the water to the air.

The light that hits the coin is bent by the water, letting you see the coin "around the corner" of the bowl. The same thing happens when you see a fish in shallow water. Refraction makes the fish look higher than it really is, something that spear fishermen have to know to hit their target.

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