To avoid this, photographers move the flash away from the camera lens. If this happens, the person in the photograph will appear to have glowing red eyes. When the bright light of a camera flash shines directly through the pupil, it can reflect off the choroid, which supplies red blood to the retina (the light-sensitive lining at the back of your eye), and bounce right back out through the pupil. The pupil of your eye is also the source of the red eyes you sometimes see in flash photographs. You may have seen a doctor shine light into the eyes of a person with a suspected head injury-they are looking at the pupillary response. Like the knee-jerk reflex, the pupillary response is used to test the functions of people who might be ill or injured. The response of the pupil is an involuntary reflex. The size of a person’s pupils can give another person a strong impression of sympathy or hostility. Not only does the pupil react to emotional stimuli, it is itself an emotional stimulus. Pupil size can change because you are fearful, angry, in pain, in love, or under the influence of drugs. The size of your pupils actually reflects the state of your body and mind. Therefore, the image of your eye is magnified twice by the magnifying glass. In this experiment, the light reflecting from your eye passes through the magnifying lens twice-once on its way to the mirror and once on its way back. That’s why the pupil of one eye can change when you shine the light into your other eye. Part of the optic nerve from one eye crosses over and couples to the muscles that control the pupil size of the other eye. More light creates more impulses, causing the muscles to close the pupil. Some of these nerve impulses go from the optic nerve to the muscles that control the size of the pupil. Light detected by the retina of your eye is converted to nerve impulses that travel down the optic nerve. Your pupil can range in diameter from 1/16 inch (1.5 mm) to more than 1/3 inch (8 mm). In dim light, your pupil expands to allow more light to enter your eye. Since most of the light entering your eye does not escape, your pupil appears black. The pupil is an opening that lets light into your eye. In a dimly lit room, open and close one eye while observing the pupil of the other eye in the mirror. Observe changes in the size of one pupil while you, or a partner, shine a light into and away from the other eye. You can see it shrink down too far, and then reopen slightly. Notice also that the pupil sometimes overshoots its mark.

Notice that it takes longer for your pupil to dilate than it does to contract.

If you are using a large mirror, bounce the flashlight beam off the mirror into your eye. If you are using a small mirror, hold the flashlight behind the mirror and shine the light around the edge of the mirror into your eye. Notice the white of your eye, the colored disk of your iris, and your pupil, the black hole in the center of your iris. You may need to adjust the position of the magnifier to get the clearest image of your eye. If you wear contact lenses or glasses, you may either leave them on or remove them.Īdjust your distance from the mirror until you see a sharply focused and enlarged image of your eye. Look into the center of the magnifying glass with one eye. Place the magnifying glass on the surface of the mirror.

One pupil larger than other