Question

How does the eye regulate the amount of light that falls on the retina?

Answer 1

Hint: Eye regulates the amount of light entering the eye's antagonistic arrangement of the circular and radial muscles which are under the control of autonomic nervous systems. In fact light entering the eye is controlled in the same way as controlled by the diaphragm of the camera.

Complete answer:
The human eyes operate on the same basic principles as a camera. A single lens focuses light from all the parts of the visual field into a sheet of light-sensitive cells. The region of the environment from which each eye collects light rays is called the visual field. This sheet of light-sensitive cell is called retina.
The bright light can seriously damage the light-sensitive cells of the retina. Thus, the intensity of light falling on the retina is controlled by the iris. It has an antagonistic arrangement of the circular and radial muscles.
The iris is a round muscle layer containing pigments that give colour to the eyes. It passes from the ciliary body through the eyeball in front of the lens. There is a hole in the centre called the pupil.
The diameter of the pupil is regulated by the muscle fibres of the iris called iridial muscle. The size or the diameter of the pupil is variable and is under autonomic control.
The amount of light entering the eye iris controlled by the radial muscles, which contract in low light, and the circular muscles, which contract in bright light.
In dim light-In dim light pupil or the apertures enlarges, letting more light to enter into the eye.
In bright light-In bright light pupil narrows down. This not only protects the interior of the eye from excessive illumination but also improves the image-forming ability and depth of eye also.

Note:
Both sets of muscles of iris are controlled by the autonomic nervous system. Sympathetic stimulation contracts the radial muscles and dilates or dilates the pupil. Parasympathetic stimulation contracts the circular muscles and constricts the pupil. Circular muscles and radial muscles are ectodermal in origin. Iris acts like the diaphragm of the camera by altering the diameter of the pupil for controlling the amount of light entering the eye in order to get the sharpest image.