Science | Science Experiment of the Week 3 | 237 - Heat Waves?

Speed of Light, Air Density - As light moves from more dense, room temperature air to less dense, hot air, it changes speed and is bent, just as it would be by a lens.

This Week's Experiment - #237 Heat Waves?

These experiments are from Robert Krampf - The Happy Scientist

I got the idea for this week's experiment while Lisa was making toast for breakfast. Looking across the top of the toaster as it heated up, I noticed the objects behind the toaster wavering in the heated air. It is one of those things that most people have seen, but usually don't think much about. Once you think about it, it really is an amazing thing to see. To experience this, you will need:

a toaster, toaster oven, or some other heat source.

Warning! We will be using the toaster as a heat source. Be sure that you have permission to use it and that an adult is around to help.

Place the toaster on a table or counter. Be sure that there is open space all around it, so nothing gets burned or melted. Since we are going to be using the toaster, you might as well make some toast at the same time, so get some butter and jelly for afterwards.

Turn on the toaster. As it heats up, you should see that looking over the top at objects across the room causes them to seem to wiggle and waver.

Why does it do that? It has to do with the speed of light. If you look up the speed of light, you will find that it is 186,000 miles per second. If you read carefully, you will see that it says that this is the speed of light in a vacuum. It travels at different speeds when it is traveling through other things, such as air, water or glass.

We are interested in what happens when light travels from one substance to another. If the transition causes the light to change its speed, and if the boundary between the two substances is at an angle that is not perpendicular to the direction the light is traveling, the beam can be bent. This is the basic idea behind how a lens works. As the light moves from the air to the curved glass and then from the curved surface of the glass to the air again, it is bent. This bending is necessary for things such as microscopes, cameras, telescopes and eyeglasses to work.

What does all of this have to do with the toaster? As the toaster heats the air around it, that air expands. This makes the air less dense and so it begins to rise upwards. As light moves from the more dense, room temperature air to the less dense, hot air, it changes speed and is bent, just as it would be by a lens. The rising air does not form a smooth, bent surface as it rises. Instead, the hot air swirls as it rises. As its surface ripples and changes, the way that it bends the light also changes. This causes the light passing through the hot air to waver and shimmer.

The same thing can often be seen in the hot air rising from a road in the summer. This bending of the light by the hot air can make it seem that there is a pool of water on the road. As your car gets closer, the pool of 'water' seems to disappear. It was not really water, but a mirage caused by the bending of light. As a child, this always delighted me, and after all these years, it still does. I may lose my hair (done that), my eyesight (new glasses) and all sorts of other things, but I hope I never lose the joy that I get from the marvels of the world around me.

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