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Penny Drop

In this activity, students learn that two eyes are better than one, especially when it comes to depth perception.

Humans have two eyes located side-by-side in the front of their heads, unlike some animals, such as horses and deer. Thanks to their close side-by-side positioning in humans, each eye takes a view of the same area from a slightly different angle. The two eye views have plenty in common, but each eye picks up visual information that the other does not.

Each eye captures its own view and the two separate images are sent on to the brain for processing. When the two images arrive simultaneously in the back of the brain, they are united into one picture. The mind combines the two images by matching up the similarities and adding in the small differences. The combined image is more than the sum of its parts – it is a three-dimensional stereo picture.

Stereoscopic vision, is used for the visual ability of creatures with forward facing eyes. The word "stereo" comes from the Greek word "stereos" which means firm or solid.

With stereoscopic vision you see an object as solid in three spatial dimensions, width, height and depth. The ability to see the depth dimension means that we can observe where objects are in relation to our own bodies with much greater precision.

Depth perception is the ability to judge objects that are nearer or farther than others. Depth perception is very important, especially when objects are moving toward or away from us in the depth dimension. When you have to cross the road and you see a car coming, you have to decide whether it is far enough away so you have time to cross, or whether it is too close and you should wait.


  • Describe how the brain helps us create images from what we observe.


  • Per Student Pair:
    small containers(plastic cups or yogurt containers)
    10+ drop objects (pennies, buttons, paper clips)
    paper and pens

Key Questions

  • Is there improvement when viewing the world with two eyes?
  • Is there improvement with the cup is closer to the subject?
  • Would a one-eyed animal have depth perception?
  • Why is depth perception important?

What To Do

  1. Divide the class into pairs and have students sit at a table. Tell them that one person will be number 1 and the other, number 2.
  2. Give each pair a plastic cup and a few pennies (or similar).
  3. Person number 1 should put the cup in front of person number 2. The cup should be about 0.5m away from person number 2.
  4. Ask person number 2 to close their right eye while number one holds a penny in the air about 0.5m above the table.
  5. Person number 1 should move the penny around slowly and person number 2 should say “drop it” when they think the penny will drop into the cup when released.
  6. When person number 2 says, “drop it,” person number 1 must drop the penny and see if it makes it into the cup.
  7. Repeat the experiment while changing one parameter at a time. For example: with the cup further away from number 2, with the cup nearer person number 2, with the left eye closed, with both eyes open. Compare the results of “10 drops” at each distance.
  8. Do the same thing with person number 2 holding the penny and number 1 guessing when to drop it.
  9. Explain that having two eyes allows humans to have vision with depth perception. This means knowing whether objects are near to us or far from us. Each of our eyes sees things from a slightly different place. This gives our brain two views of an object that it compares and uses to determine how far the object is from us. Get the students to look around and close one eye and then the other to compare the different views of their right and left eye.


  • Introduce the meaning of binocular vision (where visual fields overlap). Many predatory animals that rely on their sight to hunt their prey have their two eyes positioned on the front of their heads. This results in binocular vision and reducing their field of view in favour of depth perception. Examples include humans, owls, wolves, and snakes. Other animals, usually prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view. In such animals, the eyes often move independently to further increase the field of view. They may not have depth perception, but they can see a much broader area. This is good for an animal that is hunted by predators since it makes that animal harder to sneak up on. Examples include rabbits, horses, and deer.