This week's experiment comes from Sheldon Schafer, Director of Science Programs and Facilities at the Lakeview Museum of Arts & Sciences in Peoria, Illinois. (http://www.lakeview-museum.org) He told me that much of the USA would have a partial solar eclipse on Christmas Day and suggested this experiment as a safe and easy way to follow the eclipse. If you miss the eclipse or do not live in an area where it will be seen, there are other ways you can see how this works. I will need those, as the eclipse will not be visible from where I am. You will need:
- a solar eclipse
- a piece of aluminum foil
- a pen or pencil
- a colander or a piece of pegboard
- a smooth, flat piece of paper
When viewing an eclipse, DO NOT LOOK AT THE SUN! There is a reason for this. First, you should never look directly at the sun, as it can damage your retina. During an eclipse, the visible light is much less, but there are still plenty of harmful rays. Since the visible light is less, the pupil in your eye stays open wider, letting in more of the harmful rays, making eye damage much more likely. The same goes for filters. They may filter out the visible sunlight, giving the illusion that they are protecting your eyes, when in fact they may be letting most of the harmful, invisible rays enter the eye.
To do this safely, you want to look at an image of the sun, instead of the sun itself. We will use a pinhole, similar to the pinhole we have used in past experiments (camera obscura, paper glasses). Cut a piece of foil about 6 inches square. Use the point of the pen to make a small hole in the center of the foil. Hold this about a foot above your piece of paper, with the sun (or a lamp) shining through the hole onto the paper. This will form a circle of light on the paper. Now, you may be thinking that the round circle is due to the round hole we made. To investigate that, watch the circle of light. Hold your finger about a foot above the foil and move it around. Looking at the circle of light, you will see the image of your finger moving over it. That is similar to what will happen during the eclipse, with the moon taking the place of your finger.
To see this with many images, hold the colander about a foot above the paper, letting the sunlight shine through the holes. On the paper, you will see lots of tiny circles of light. Now you might think that the dots were round due to the round shape of the holes. Many colanders have square holes, but you still get round circles of light unless you hold the colander very close to paper. What you are seeing is an image of the sun (or the light bulb in the lamp). As we have seen in past experiments, a small hole can act as a lens, either letting you see more clearly, or focusing an image as with a camera obscura. We are doing the same thing with multiple holes to give multiple images. As the eclipse begins, you will notice something happening to the circles of light. You will see the shadow of the moon slowly eat away into the side of each circle.
I first saw this in a much more amazing way. I was walking in the woods during an eclipse. If you have walked in the woods, you will notice that the light shining through the leaves forms lots of small circles of light on the forest floor. The spaces between the leaves form the "pinholes". That day, the forest floor was covered with crescents of light, letting me safely follow the eclipse while continuing my hike.