The colored part of the eye that constricts and dilates to let light in.

• A. Pupil
• B. Iris
• C. Retina
• D. Sclera

Answer: (B)

The iris is the colored part of the eye that controls how much light enters. It does this with two layers of smooth muscles: the sphincter pupillae constricts the pupil to make it smaller in bright light, while the dilator pupillae dilates the pupil to let in more light in dim conditions. By adjusting the size of the pupil, the iris regulates the amount of light that reaches the retina, where photoreceptors detect it. The retina is the light-sensitive tissue at the back of the eye, and the sclera is the white protective outer layer. The pupil itself is the opening that light passes through, so the iris indirectly governs light entry by changing pupil size.