Roll a sheet of white paper lengthwise to form a round tube approximately 1 inch (2.5 cm) in diameter.
Secure the edge with transparent tape to keep the tube from unrolling.
Do the same with the other sheet of white paper, so you have two tubes of equal size.
To Do and Notice
Put one tube up to each eye and look through them at the white screen, wall, or sheet of paper. First close one eye, and then open it and close the other. Does the brightness of the spot appear the same for each eye?
Keeping the tubes in front of your eyes, angle the ends of the tubes closer together so the two spots you see overlap slightly. Notice that there is a brighter area where the two spots overlap.
Now, overlap the spots completely. Does the combined spot look brighter than either spot alone? Find out by closing one eye.
What’s Going On?
When you partly overlap the two spots, your open eye and brain conclude that the sum of the two spots of light should be brighter than one spot alone. If the spots overlap completely, it’s more difficult to perceive the brightening.