By Howard J. Bennett

After you read this paragraph, find a mirror and take a close look at your eyes. What you’ll see staring back at you are two amazing, almond-shaped structures. The white of the eye is called the sclera. This is the tough outer tissue that gives the eye its strength. The colored part is the iris. For all its beauty, the iris is just a muscle that controls the amount of light that enters the eye. The black area in the center of the iris is the pupil. The pupil is not a structure at all, but merely an opening in the center of the iris.

If you’d like to learn more about what the pupils do, pick a partner and do the activities that follow.

What Happens to Pupils In a Dark Room?

When you first enter a dark room, you can barely see anything. After a few moments, your brain sends signals to your iris telling it to open and let in more light. That’s why you can see a lot more after you’ve been in the room for five minutes or so. If you use a low-wattage flashlight to look at someone’s eyes in a dark room, you will notice that the pupils are huge.

Are All Pupils Equal?

About one in five people have pupils that are not the same size. This is true for many of our paired body parts, like hands, feet, ears and the ones nobody sees. But unequal pupils have a really cool name: anisocoria (a-NEE-so-KOR-EE-a).

What Happens to Pupils If You Get Excited?

If you get excited, a number of changes take place in your body. Your heart rate and blood pressure increase. You become more alert. And your pupils get bigger. This happens because your nervous system knows something important is happening and your eyes need extra light to see well.

What Happens to Pupils If You Shine a Light in Your Eyes?

If you’ve been paying attention, you should know the answer to this one right away. If a bright light hits your eyes, your pupils become smaller because you don’t need that much light to see. Ah, but this one has a twist. Because bright light can damage the eye, your pupils are also getting smaller to protect your retina. The retina is the part of the eye that processes visual information and sends it to the brain.

To demonstrate this mechanism, have your partner put one of his hands on the bridge of his nose to “separate” his eyes. Shine a light in your partner’s right eye. Notice that the pupil gets smaller. Now, shine the light in his right eye again, only this time look at his left eye as you do it. What happens to the left pupil? It got smaller at the same time the right one did. That’s because both eyes protect themselves even if the light is only in one eye. Pretty cool, huh?