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Science Snacks
Science activity that demonstrates a visual illusion
Science activity that demonstrates a visual illusion
  • Science activity that demonstrates a visual illusion
  • Science activity that demonstrates a visual illusion

Overlapping Spots

Make a bright spot brighter.

You have two eyes, yet you see only one image of your environment. How does your brain interpret brightness when each eye perceives it separately?


Grade Bands: 
3-5
6-8
9-12
Subject: 
Biology
Anatomy & Physiology
Neuroscience
Perception
Light, Color & Seeing
Optical Illusions
Physics
Light
Keywords: 
eye
paper
NGSS and EP&Cs: 
LS
LS1
PS
PS4
CCCs
Cause and Effect
Structure and Function

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Tools and Materials

  • Two sheets of 8 1/2 x 11-inch (21.5 cm x 28 cm) or A4 white paper
  • Transparent tape
  • A well-lit white screen, wall, or sheet of paper

Assembly

  1. Roll a sheet of white paper lengthwise to form a round tube approximately 1 inch (2.5 cm) in diameter.
  2. Secure the edge with transparent tape to keep the tube from unrolling.
  3. 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.



Related Snacks

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Hole in Your Hand

What happened to your whole hand?

Science activity that demonstrates the dominant eye phenomena
Circles or Ovals?

Your brain combines information from your eyes in surprising ways.

Science activity that demonstrates lateral inhibition in visual perception
Lateral Inhibition

Which is in control, your eyes or your brain?



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Attribution: Exploratorium Teacher Institute

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