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Asked by: Molly Hatala
School:Binghamton West Middle School
Teacher:Jo Ann Summerlee
Hobbies/Interests:Dance, basketball and guitar
Career Interest:Medicine


Answered by: Donna M. Morgan, PHD, RN
Title:Clinical Assistant Professor, Binghamton Universit
Department:Decker School of Nursing
About Scientist:Research area:
Heart disease in women

Diploma in Nursing, Methodist Hospital in Philadelphia; BS in Nursing, Elmira College; MS in Nursing, Syracuse University; and PhD in Nursing, Binghamton University.


Date: 09-20-2007

Question: Why do our pupils get bigger and smaller?

Answer: Light is needed to be able to see. We can't see anything in total darkness, but we know the things are still there. Just because we can't see the wall in front of us in total darkness, we'll still bump into it!

The pupil is the round black space in the center of the eye. The pupil lets different amounts of light get into the eye. Muscles in the iris, the colored part of the eye, control the size of the pupil.

Normal pupils are round and will constrict, or get smaller, when a bright light is shined in the eye. The size of the pupil is measured in millimeters (mm). Very small pupils, or 1 mm, are sometimes called pinpoint pupils. The biggest, or most dilated, pupils can get is 10 mm. About one out of five people have pupils that are not the same size, but the pupils still change the same.

When we're in someplace that is dark, like a movie theater, our pupils dilate, or get bigger. This lets more light get into our eyes so we can see. When we're in someplace that is very bright, like outside on a sunny day, our pupils constrict, or get smaller. This is so too much light does not get into our eyes.

There are other reasons that cause our pupils to change size. Pupils dilate when we are scared, like being chased by a bear! We need to see the best we can to get away. Some medicines given to us by a doctor will make our pupils constrict or dilate. Some illegal drugs, like cocaine, will make out pupils constrict. The pupils may be different sizes when the brain has been injured.

Try this experiment. You will need a room that is a little dark, a mirror, and a flashlight. The room needs to have enough light so you can see the size of your pupils in the mirror. A bathroom works well. Stand in the room about 1 minute so your pupils will adjust to the darkness, then look at the size of your pupils. Now, shine the flashlight in your eyes and see how quick your pupils constrict! When the room was dark, you needed bigger pupils so you could see. A room that is bright does not need the pupils to be wide open to see.

Ask a Scientist appears Thursdays. Questions are answered by faculty at Binghamton University.  Teachers in the greater Binghamton area who wish to participate in the program are asked to write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail Check out the Ask a Scientist Web site at To submit a question, download the submission form(.pdf, 460kb).

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Last Updated: 6/22/10