Learn About Straight-line Light

by Lester Showalter | Sep 1, 2023 | 0 comments

When I was a boy, I enjoyed making tents. I liked to use covers to make cozy tents with rooms and doorways. So if you enjoy making tents too, you will enjoy this activity. I will give you an educational reason for making a tent.

You will need a small table such as a card table. From one edge of the table, hang a large sheet of white paper. A white cloth sheet will work, but smooth white paper will be better. From the opposite side of the table, hang a large piece of heavy cardboard. The side of a refrigerator or stove packing box would be just the thing.

In the center of the cardboard, cut a hole about 2 inches (5 cm) in diameter. Now cover the hole with a piece of aluminum foil. In the center of the aluminum foil, punch a hole about the size of a lead pencil.

Cover the entire table with heavy covers that will not let any light in. Leave only the hole in the aluminum foil uncovered. The covers must come down to the ground all around. The darker the inside of the tent, the better.

Make the tent so you can lift one side of the blankets and crawl into the tent to do your experiment. Sit with your back to the cardboard, but to the side of the hole. What do you see on the white paper?

If you have never seen this before, you have a real surprise waiting for you. But since I want to tell you why it happens, I will tell you what you will see. On the white paper, you will see a picture of what is outside the cardboard.

The really surprising thing is that the picture is upside down and backward. The sky will be at the bottom and the ground will be at the top. The houses will be hanging upside down from the ground. If any cars or people go by, they too will be upside down.

The reason for this strange world on the paper is that light travels in straight lines. The light from a spot in the world outside your tent goes in a straight line through the hole and lands at a spot on the paper. So what is up lands at a spot that is down on the paper. What is to the right lands at a spot that is on the left of the paper.

Your upside down world will be fuzzy, but you should have no trouble deciding what the things are that you see. You can experiment with different size holes in the aluminum foil. A small hole will make a sharper picture, but it will not be very bright. A larger hole will make the picture brighter, but it will be more fuzzy. This activity will work best on a bright, sunny day.

Now I have another surprise for you. What you see in your tent is what happens in your eyes. Each eye is like a round room. The hole in the front of your eye, called the pupil, lets light into the eye. The picture that comes through the pupil is upside down on the back of the eye. You are actually seeing upside down! But you know it does not look upside down. Everything looks right side up. That is because your brain turns it upright again. This is part of the wonderful way God made your body.

straight line light illustration
Photo © Kevin Shank.

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