What structure in the eye is responsible for regulating the amount of light that enters through its opening?

The iris is the structure in the eye that is responsible for regulating the amount of light that enters through its opening, which is known as the pupil. The iris is a muscular ring of tissue located behind the cornea and in front of the lens. It can contract or expand, thereby adjusting the size of the pupil. When there is bright light, the iris contracts to make the pupil smaller, reducing the amount of light that enters the eye to protect the retina from damage and to improve visual acuity. Conversely, in low light conditions, the iris relaxes to widen the pupil, allowing more light to enter to enhance visibility.

In contrast, the pupil itself is simply the opening created by the iris, while the lens focuses the light that has entered the eye onto the retina at the back of the eye. The retina contains photoreceptors that detect light and are responsible for converting it into neural signals for the brain to interpret as images.