What changes take place in the shape of an eye-lens: (a) when the eye is focused on a near object? (b) When the eye is focused on a distant object?

Hint: To solve this question one needs to understand the function of the ciliary muscles. These muscles' primary function is accommodation. That is the adjustment of the eye to view nearby and distant objects. Based on the contraction and relaxation of the ciliary muscles the focal length also varies which enables one to see objects near or far.

Complete step by step answer:
The eye lens is made up of fibrous and jelly-like substances. It is held to the walls of the eye through muscles known as ciliary muscles. These ciliary muscles can modify the curvature of the lens to some extent.
a. When the eye is focused on a nearby object or that which is close to the eye, the contraction of the ciliary muscles takes place. As a result of the contraction by the muscles, the curvature of the eye lens increases. Therefore the lens becomes thicker. Hence, this leads to a decrease in the focal length of the eye. This process assists us to view nearby objects.
b. When the eye is focused on a far-way object, the ciliary muscles relax thereby the eye lens becomes thin. This change, therefore, increases the focal length of the eye lens. Hence, aids one to observe distant objects.

Note: The eye lens of a human eye forms an inverted and real image of the object on the retina. The lens's main function is the adjustment of the focal length to focus the objects at various distances on the retina. Accommodation is the ability of the eye lens to adjust the focal length. Focal length measures how powerfully an optical system diverges or converges light.