Which part of the eye can change in size to let more or less light enter?

The iris is the part of the eye that plays a crucial role in regulating the amount of light that enters the eye. It is a pigmented muscle located around the pupil, which is the opening in the center of the iris. When the light levels are bright, the iris constricts, making the pupil smaller and allowing less light to enter. Conversely, in dim light conditions, the iris relaxes, enlarging the pupil to permit more light to pass through. This adjustment helps to protect the sensitive retina from excessive brightness while ensuring adequate light reaches it for clear vision.

The retina is responsible for detecting light and converting it into neural signals, but it does not control the amount of light entering the eye. The cornea helps to focus light as it enters the eye, but it does not change size. Similarly, the lens adjusts its shape to focus light onto the retina but does not change in size to regulate light entry. Thus, the iris is the only part that directly changes in size to control light levels entering the eye.