Iris and Pupil

What are they?

The iris is the part of the eye that identifies your eye color, and is shaped like a flat donut. It contains the melanin, a brown pigment. The more you have, the darker you eyes appear. Blued-eyed people have melanin at the back of the iris, but barely anywhere else, while brown-eyed people have plenty of melanin scattered about the iris. Albino people (pink-eyed, white skinned people), on the other hand, lack most or all pigment in their bodies. The reason why their eyes appear pink is that the light reflects off the back of the eye, where blood vessels are, making them reddish-pink in appearance. The pupil is simply the hole in the middle of the iris that light passes through.

Two sets of muscles control size of the iris. One is like a sphincter circling the pupil, to make the hole smaller. The set radiates out like the spokes of a bicycle, enabling it strength to widen the pupil.

What do they do?

The iris controls the amount of light that enters the eye. If you look closely at someone's eyes, while shining a flashlight at it, the pupil becomes smaller. Even if you shine light in one eye, the other iris will also change in unison. This is because the eyes do not like abrupt changes in light, and therefore, has the iris to constrict to allow less light in.

Another interesting fact is that the iris reacts to emotions. Say someone unexpectedly shows you a picture of a murder scene. Your first reaction is that you are interested in trying to figure out what the photo is. At this point, your iris constricts to widen your pupil. It finally hits you that the picture is terrifying, and you shun it away, while your iris narrows the pupil, allowing less light in. Something interesting will make you open your eyes to allow more light in.