What part of the eye controls the amount of light entering by changing the size of the pupil?

The iris is the part of the eye that controls the amount of light entering the eye by changing the size of the pupil. It functions as a diaphragm, expanding or contracting in response to the brightness of the light in the environment. When in bright light, the iris constricts, reducing the diameter of the pupil to limit the amount of light that enters, which helps protect the retina. In low-light conditions, the iris dilates, allowing more light to enter through a larger pupil, which improves vision in dim lighting.

The ciliary body is involved in focusing the lens and adjusting its shape for near and distant vision, but it does not directly control the size of the pupil. The sclera is the white, outer layer of the eyeball that provides protection and structure but has no role in light regulation. The cornea is the clear front surface of the eye that helps to focus light but is not involved in changing pupil size. Therefore, the iris is specifically responsible for adjusting pupil size, making it the correct answer.