The Iris

eye · sight · body systems

Colorful controller. The iris is the colored part of your eye that gives you blue, brown, green, or other eye colors. It's made of tiny muscles that automatically squeeze or relax to change the size of your pupil, the dark circle in the middle.

Light gatekeeper. When light is bright, your iris muscles automatically make the pupil smaller to protect the sensitive parts of your eye. When it's dark, the iris relaxes and the pupil gets bigger so more light can reach the back of your eye.

Unique as fingerprints. Every person's iris pattern is completely different, kind of like fingerprints. No two people have exactly the same iris pattern, which is why some security systems use iris scanners to identify people.

Instant reactions. Your iris responds in just a fraction of a second, even before your brain realizes the light has changed. This automatic response is called a reflex, and it protects your eyes without you having to think about it.