Which structure controls the amount of light entering the eye?

Which structure controls the amount of light entering the eye?

• A. The iris
• B. Focuses light onto retina
• C. Detects light
• D. Maintains the eyeball shape

Answer: (A)

The amount of light entering the eye is controlled by the size of the pupil, which is adjusted by the iris. The iris acts like a tiny adjustable diaphragm around the pupil, using two sets of muscles to change its size: in bright light, the iris constricts the pupil to limit how much light gets in, protecting the inner eye from glare; in dim light, it dilates the pupil to allow more light in so you can see better. This regulation happens before light is focused or detected, so the iris is the key structure for controlling light entry. The other roles you might think of—focusing light onto the retina, detecting light, or keeping the eyeball's shape—are performed by other parts: the cornea and lens focus light, the retina detects light with photoreceptors, and the eyeball’s shape is maintained by the outer layer and supporting tissues.

The amount of light entering the eye is controlled by the size of the pupil, which is adjusted by the iris. The iris acts like a tiny adjustable diaphragm around the pupil, using two sets of muscles to change its size: in bright light, the iris constricts the pupil to limit how much light gets in, protecting the inner eye from glare; in dim light, it dilates the pupil to allow more light in so you can see better. This regulation happens before light is focused or detected, so the iris is the key structure for controlling light entry.

The other roles you might think of—focusing light onto the retina, detecting light, or keeping the eyeball's shape—are performed by other parts: the cornea and lens focus light, the retina detects light with photoreceptors, and the eyeball’s shape is maintained by the outer layer and supporting tissues.