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Those Reflexive Pupils


You and Your Students!

Script By

Vicki Cobb, Education World Science Editor


Discover the pupil reflex.


Human Body

Required Props

  • flashlight
  • computer screen or TV screen

Setting the Scene (Background)

Have you ever noticed that it can take a few minutes for your eyes to adjust to a dark movie theater when you walk into one on a bright, sunny day? Have you ever noticed how yours eyes "hurt" if you turn on a light when you wake up in the middle of the night? In both of those cases, your eyes' pupils must adjust to the amount of light that enters them.

When you are in the dark your pupils are large -- letting in as much light as possible. In bright light, your pupils are small, cutting down on the amount of light that enters the eyes. The opening and closing of the pupil is a reflex, it requires no thought -- a concept that might intrigue your students.

Stage Direction

Show-Biz Science is scripted by popular children's book writer Vicki Cobb. Click to learn more about Vicki or to read a brief synopsis of her philosophy of teaching science.

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Have students work in pairs for the first activity (Act I) below. The second activity (Act II) is a whole-class activity.


Act I
With students working in pairs, provide one partner with a flashlight. Turn out the lights in the room and wait a few minutes to so students' eyes can adjust to the dimness. Have student pairs look into each other's eyes to see how large the pupils are. Have one student shine the flashlight into the eye of the other student and observe what happens to the pupil. (It suddenly gets smaller.) Give all students an opportunity to witness this.

Act II
Turn out the classroom lights. Turn on a TV or a computer monitor. Have your students watch the screen for at least three minutes. Then, turn on the lights again. Ask your students what happens to the screen. (It suddenly appears dim.) Why?

Behind the Scenes

A reflex is an instant response that happens without thinking. The pupil reflex was discovered in 1751. It was the first reflex of the human body to be discovered. The pupil's changing size is the result of the opening or closing of the iris (the colored part of the eye) as it controls the amount of light that enters the eye.

In the second activity, the pupils open wide in the dimly lit classroom. Most of the light entering the eye comes from the screen. When the light is turned on, the pupil gets smaller to cut down on the light entering the eye. As a result, less light reaches the pupils from the screen; the screen appears to be dimmer.

Article By Vicki Cobb
Education World®
Copyright © 2009 Education World

Originally posted 09/15/2005
Last updated 01/19/2009