You and Your Students!
Vicki Cobb, Education World Science Editor
Watch TV upside down on a piece of paper.
Physical Science, Light
Setting the Scene (Background)
A magnifying lens is good for more than just making images larger. You can use one to project an image upside down on a piece of paper. All kinds of images! But the most interesting images will be moving ones from a TV. Your students will have a lot of fun discovering that they can use a magnifying lens to watch TV (or a computer screen) upside down on a piece of paper.
This activity is best done in small groups. You can demonstrate how it's done. Then let your students experiment with it.
Turn off the lights, tune the TV to a kid-friendly show, and stand back about 10 feet from the set. Hold a magnifying lens in one hand and a sheet of paper in the other. Position the lens vertically between the TV and the paper, and hold the paper about 6 inches from the lens. (Both the lens and the paper should be parallel to the TV screen and at right angles to the floor.) Move the paper away from and toward the lens until you see a focused image of the television picture. Sit back and watch the show -- but you may have to turn your head upside down, because the image is inverted and backward!
If you are not satisfied with the size of the image, you can change the magnification by changing your distance from the TV. Try standing 20 feet from the TV. Then try 2 feet.
Behind the Scenes
When light leaves a point on the TV, it spreads out in many different directions. As a result, light from one point on the TV hits the entire surface of the magnifying lens. The lens then bends and redirects this light so that it comes back together again as a single point. That happens to every point of light that makes up the image on the TV screen. In that way, the TV image is reconstructed by the lens.
The lens bends light from the left portion of the TV, redirecting it to the right portion of the image; and light from the right portion of the TV is redirected to the left. Light from the top of the TV gets redirected to the bottom of the image, and vice versa. As a result, the image is upside down and backward.
You may notice that when you are closer to a TV, you have to hold the paper farther from the lens to find a clear image than when you are farther away from the TV.
This experiment is from my book Light Action! Amazing Experiments with Optics. This book is being re-released by The International Society for Optical Engineering.
On a personal note, this book is a family project. My younger son, Josh, is an optical engineer and my coauthor. Theo, my eldest, is an artist, and he did the illustrations.
Article By Vicki Cobb
Copyright © 2005 Education World