What defect of vision can be rectified by
a) by using a concave lens
b) by using a convex lens

Hint: Whenever two or more lenses are used close to each other we can treat them as one lens with power equal to the sum of powers of the lenses involved. So here we will predict the change using one kind of lens will make on the image formed.

Formula used:
$P={{P}_{1}}+{{P}_{2}}$

Complete step by step answer:
The lens in our eye focuses the light at our retina. Using this information, we deduce that the lens in the eye must be a converging lens or a convex lens. The lens in our eye must have a very short focal length so that the image is formed on the retina located at the back of our eyeball. Using this we know that the power of the lens in the eye will be a large positive number.
(a) So, when a concave lens is used, the total power will decrease as the power for a concave lens is negative. This will increase the focal length of the eyeball. This is useful when the person has a difficulty in seeing objects located far away and the image is formed behind the retina. This defect of vision is known as myopia or nearsightedness.
(b) When a convex lens is used, the total power will increase as the power for a convex lens is positive. This will decrease the focal length of the eyeball. This is useful when the person has a difficulty in seeing objects located near to the eye and the image is formed in front of the retina. This defect of vision is known as hyperopia or farsightedness.

Note:
Although there is a significant distance between the lens in the eye and lens used for glasses, we can take them to be near to each other for this question as the broad result in the change of power would be the same. Increasing the distance between them will decrease the magnitude of change in power a bit but the overall effect will be the same.