What Causes Dilated Pupils?

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Updated on Aug 09, 2013

Your eyes are amazing organs. Look in the mirror, and you’ll see three colors in your eyes: the white part (sclera), the colored part (iris), and a small black dot (the pupil).

The iris helps control the movement of your pupil. It’s sort of like a lens on a camera: it moves in and out, changing how big or small your pupil is. Without your iris controlling the size of your pupil, seeing in different situations would be difficult! So how come? To find out, we’re going to build models of the pupil and see how each affects the amount of light let in.


How do different pupil sizes change the way light enters the eye, and what does this tell us about why our pupils change sizes in the first place?


  • 5 sheets of card stock paper
  • 5 concentric circle shapes to trace, with diameters of 1 cm, 2 cm, 3 cm, 4 cm, and 5 cm
  • Scissors
  • Flashlight


  1. Place the smallest circle on one sheet of card stock paper.
  2. Trace it.
  3. Use your scissors to cut out the shape you traced.
  4. Repeat steps 1-3 for each circle shape.
  5. Once you’ve assembled all your circles, think about how your pupils change in different situations. Your pupils get smaller when you’re in bright environments, and they shrink when you’re in the dark. Why do you think our eyes do this? Use this brainstorm to make some predictions about what you think will happen when you conduct your experiment.
  6. Stand in a dark room. Hold the paper with the biggest hole about six inches away from the wall.
  7. Shine your flashlight through the paper. How much light makes it through to the wall? This is your way of modeling how much light your pupil lets into your eye.
  8. Sketch how much light shines through to the wall in your notebook.
  9. Repeat with each circle, working from biggest to smallest in order.
  10. Make sure you record how much light shines through each time!


You should have found that the biggest circle let the most light shine through. As your circle got smaller, less light made it to the wall.


The reason you got the results you did was because the smaller the hole, the less light is able to pass through. This is also true of your eyes’ pupils. When you’re out in the sun, your eye has to deal with a lot of light! Your pupil gets smaller in order to protect your vision, and lets in just enough light for you to see. When you’re in a darker room, your pupil needs to let more light in so that you can see your surroundings, so naturally, it gets bigger.

If you want, you can learn more about your pupils by changing up the experiment. Working in groups of two, sit in a bright room and a dark room for a few minutes to observe what happens to your pupils in each situation. Remember to make a hypothesis—you can base it on what you’ve already learned!

It might also be fun to think about the pupils of other mammals. Have you ever seen a cat’s pupils when the cat is startled? Why do you think this is? Also, think about how the eyes of cats and other nocturnal animals shine at night. Do you think you can find out why? Whatever you set out to explore, have fun. Science is a never-ending adventure that is only limited by your imagination.

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