Which part of the eye is responsible for adjusting the pupil size to control light entry?

The iris is the part of the eye that is responsible for adjusting the pupil size, thereby controlling the amount of light that enters the eye. It is a muscular structure located behind the cornea and in front of the lens. The iris can contract or expand in response to light conditions; in bright light, it contracts to reduce pupil size and limit light entry, while in dim conditions, it expands to increase pupil size and allow more light to enter.

Understanding the function of the iris is crucial, especially in contexts like night vision, where managing light exposure is essential for effective vision under low-light conditions. The pupil itself is simply the opening created by the iris and does not actively control size; that function lies with the iris. The lens focuses light onto the retina, while the retina is responsible for converting light into neural signals, not for controlling light entry.