Which part of the eye is primarily responsible for controlling light entry through pupil size adjustment?

The iris is the part of the eye that is primarily responsible for controlling light entry through the adjustment of pupil size. It functions as a muscular diaphragm that can expand or contract to change the diameter of the pupil. This adjustment is crucial for regulating the amount of light that enters the eye, helping to protect the inner structures from excessive brightness and also enhancing vision in varying light conditions.

When exposed to bright light, the iris contracts, resulting in a smaller pupil that reduces the amount of light entering the eye. Conversely, in low light conditions, the iris dilates, increasing the pupil size to allow more light in for better visibility. This reflexive control helps maintain optimal conditions for vision and contributes significantly to the eye's overall functionality.

While the retina is responsible for detecting light and sending visual signals to the brain, the cornea primarily plays a role in refracting light to focus it onto the retina, and the lens is involved in fine-tuning that focus. However, neither the cornea nor the lens has a direct role in regulating pupil size. Thus, the iris is indeed the correct answer as it directly controls the amount of light that reaches the retinal surface.