Question

Name the type of lens used to correct this defect.

Answer 1

The eye defect mentioned in the figure is called myopia. Myopia is also called short-sightedness wherein the person is clearly able to see the things that are nearby him, but cannot clearly perceive things at a far distance.

Complete step by step answer:
Our eye lens is very versatile and effective. When we have to perceive the things that are in our vicinity, the eye lens becomes bulged and whenever we have to perceive the things that are in a very far distance, the eye lens flattens itself.
However, due to some defect, the eye lens, when it bulges when it perceives a nearby object, does not return into its original flat state even after taking the eyes off the nearby object. This permanent bulging of the eye lens results in an eye defect known as myopia or short-sightedness.

From the image, it is clear that due to the bulging of the eye lens, the image of a far object formed by the lens does not fall exactly on the retina and falls in front of it, thereby, making it difficult to perceive the image properly.
To correct this defect, one has to use a corrective lens. This lens should be biconcave in nature with a negative power (or negative focal length).
The Biconcave lens is a diverging lens. This ensures that the incoming rays are diverged to some extent, before entering the eye lens so that the image falls exactly, on the retina, as shown:

As you can see in the image, the usage of a biconcave lens will ensure the image falls correctly, on the retina, thereby making the perception of the image clearer and better.

Note:
This defect of myopia is common among all age groups. However, there is an opposite defect of this, called hypermetropia or long-sightedness. This is more common in older ages, post 40 years when the eye muscle loses its elasticity and it flattens so much that the image is formed behind the retina, which is like an extremely opposite case of this defect myopia.
To correct that defect, a biconvex lens of positive power should be used.