Which part of the human eye adjusts the size of the pupil?

The iris is the part of the eye responsible for adjusting the size of the pupil. It is a thin, circular structure located between the cornea and the lens of the eye. The iris contains muscles that expand and contract in response to light intensity. In bright light conditions, the muscles constrict to make the pupil smaller, reducing the amount of light that enters the eye. Conversely, in low light conditions, the muscles relax to allow the pupil to enlarge, increasing light intake to enhance vision.

The other structures mentioned have distinct functions; for example, the cornea helps to focus light as it enters the eye, the lens fine-tunes focus for near or distant vision, and the retina converts light into neural signals for the brain to interpret images. This functional differentiation highlights the specific role of the iris in controlling pupil size.