Question

I have seen in some books that pupils control the amount of light entering in our eyes. Is this correct? Explain to me please.

Answer 1

Hint: The pupil is the hollow blackish part in the middle of the iris through which light enters into the eye and strikes the retina.

The constriction and dilation of the pupil with respect to the amount of light entering the eyeball is a natural reflex. During the day, or when we are in a bright environment, the pupil constricts which allows a small amount of light rays to enter the eye. Similarly, during twilight conditions, the pupil enlarges to allow a larger amount of light to enter into the eye.

Complete answer:
The pupil is actually a small hole in the eyeball that allows entry of light. It is guarded by the iris, which is the extension of the choroid portion of the eyeball in the front. The iris has the muscular fibers- the ciliary and the radial, which helps in the regulation of the diameter of the pupil, or in clinical terms, the aperture.
During daylight, the ciliary or circular muscles of the iris contract, allowing the iris to relax and in turn decrease the aperture of the pupil. The condition is just the reverse at low light conditions. At night, the radial muscles of the iris contract that forces the iris to contract which in turn increases the aperture of the eyeball.

Note:
Confusion might arise sometimes as students may think that it is actually the iris that controls the amount of light entering into the eyeball rather than the pupil because iris is the part that consists of the muscular fibers that in turn regulate the diameter of the pupil and the pupil is just a hole in the iris.
However, it should be remembered that the iris is just the colored portion of the eyeball and has nothing to do with the light regulation. Thus, whenever asked, the answer should be pupil.