Sunlight, Energy, and Crayons

If you are lucky enough to have some snow, this is a simple experiment you can use to learn about sunlight and energy. It fits nicely into discussions of solar energy and how energy changes from one form to another.

To try this, you will need:

  • snow
  • several colors of crayons
  • a sunny day

Select crayons of different colors, making sure that at least one of them is white, and at least one of them is black. Peel the paper cover away from the crayons, as in the photo above.

Find a flat, sunny patch of snow, and place the crayons gently on the surface. Write down the starting time, and you may want to take a photo of the start, as I did with the photo at the top of this page.

After 15 minutes, check to see if anything has changed. Write down your observations. Keep checking every 15 minutes or so.

This photograph was taken 30 minutes after I placed the crayons on the snow. Notice that while the white crayon is still on the surface, the others have all melted slightly into the snow, and the black crayon has melted the deepest hole.

After two hours, the white crayon was still on the surface, but the black crayon had melted a hole that was over three inches deep! The other colors had melted some of the snow, but not nearly as much as the black crayon.

Why?

The colors that we see are the result of light reflecting from an object. White light is made up of all the colors of the rainbow, mixed together. If an object absorbs all of the colors except for red, and reflects the red light, then that object looks red to us. If the object absorbs all of the light except for the blue, then the blue light is reflected, making the object look blue. An object that absorbs all of the colors will look black, and an object that reflects all of the colors will look white.

But why would that cause the black crayon to melt the snow faster? One of the interesting things about energy is that we cannot get rid of it. It never goes away, but it may change from one kind of energy to another. The red crayon reflects the red light, and absorbs the other colors. That absorbed light energy is converted into heat, making the crayon warmer. Since the light energy that is absorbed is changed into heat, the crayon that absorbs the most light will be the warmest. Since the black crayon absorbed the most light, it had more heat energy to melt the snow.

On the other hand, the white crayon reflected most of the light, and absorbed very little. That means that it did not have much light energy to convert into heat, so it did not melt the snow.

This idea has some very useful applications. Imagine that you wanted to use solar energy to heat water for your house. You would want the container that held the water to absorb as much light energy as possible, so it could convert that light into heat. What color should it be?