Light enters inside the side through a small opening in the iris of the eye which is called the Pupil.
The outer most part of the eye Cornea and the outer chamber aqueous humor are both transparent and allow light to refract through them.
The Iris, which a circular and thin comes next to the cornea and aqueous humor. It is reponsible for regulating the amount of light that can reach the retina by altering the pupil size.
The change in the size of the pupil is mediated by muscles in the iris.
There are two types of muscles, radial muscles and circular muscles.
In bright light, the radial muscles of the iris relax while the circular muscles contract. This decreases the pupil size allowing less light to pass into the retina.
In dim light, radial muscles of the iris contract while the circular muscles relax. This increases the pupil size allowing more light to pass into the retina.
