You know that myopia is a common refractive defect of vision. A person with this defect can clearly see only objects that are near. Using concave lens of suitable power this defect is corrected.

i) Mention the other two types of defects.

ii) Explain how they can be corrected.

Myopia or near sightedness is an eye defect in which a person can see clearly only when the objects are near. This happens when the lens becomes too thick and the light rays undergo greater refraction which makes the light rays to converge before the retina, making the eyesight blurry.

Using a concave lens can be helpful as it concentrates the light rays on the retina. The below image explains the process

The two other types of defects are:

• Hypermetropia

• Astigmatism

Hypermetropia or far sightedness is an eye defect there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hypermetropia can be corrected by wearing glasses/contacts that contain convex lenses.

Astigmatism is an eye defect in which the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. Astigmatism can usually be corrected by using a special spherical cylindrical lens.