Question

Ciliary muscles of human eye can contract or relax. How does it help in the normal functioning of the eye?

Answer 1

The ciliary muscles of the human eye can contract or relax the eye lens and help in changing the focal length of the eye lens. Because of this, the eyes can focus the images of nearby as well as distant objects on the retina.
We can understand this by following examples:

(a) When the eye is focussing on a distant object. In this case, the ciliary muscles are completely relaxed. The relaxed muscles tightly pull the suspensory ligaments, which in turn pull the eye lens, because of which the eye lens becomes thin. The thin eye lens has a small converging power, which is sufficient to converge the parallel light rays from the distant object to form an image on the retina.

(b) When the eye is focussing on a nearby object. In this case, the ciliary muscles contract, because of which they cannot pull the suspensory ligaments tightly. As the suspensory ligaments are loose, they stop pulling the eye lens, and therefore the eye lens bulges and becomes thick. The thick eye lens has greater converging power, which helps in converging the diverging light rays from the nearby object to form an image on the retina.