Question

Describe and explain, how a normal eye can see objects lying at various distances clearly.  

Answer 1

A normal eye can see objects situated at various distances clearly because of its power of accommodation. Power of accommodation is the ability of the eye to focus nearby as well as distant objects on the retina by changing the focal length of the eye lens. The ciliary muscles can contract or relax the eye lens and hence help in changing its focal length.

(a) When the eye is focussing on a distant object, it is said to be unaccommodated. The ciliary muscles are relaxed, because of which they tightly pull the suspensory ligaments, which in turn pull the eye lens. The eye lens becomes thin and its converging power becomes small. The small converging power 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, it is said to be accommodated. 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, because of which the eye lens bulges or 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.