Question

Astigmatism occurs when the cornea does not have a truly spherical shape. This defect can be cured by the used of:
(a) Concave Lens
(b) Cylindrical Lens
(c) Convex Lens
(d) Plano-convex Lens

Answer 1

Hint: This optical defect occurs as the light entering through the eye cannot be focussed at a point.
This is also known as refractive error. Just like the other visual effects this one can be corrected/ cured by using a lens that will behave as the cornea for the eye.

Complete step by step answer:
As the name suggests “refractive error”, this optical defect is due to uneven refraction.

Astigmatism is an optical error that occurs as the eye does not have a truly spherical shape. In a normal eye, free from any defect, the cornea is a perfect sphere.

Therefore, when the light enters the eye, the eyeball being perfectly round in shape, helps the light to refract evenly and the object is focussed at a particular point.

However, in case of an eye, having Astigmatism, the eyeball is not a perfect sphere as result of which

The light refracts unevenly at different points, bending more at one point compared to other points, as a result of which the object is not focussed on a particular point.

This gives a blur and unclear vision. Also can cause eye-strain.

Thus, the distorted shape of cornea is responsible for the uneven bending, which in turn causes Astigmatism.

In order to correct this defect, the light must be focussed at a particular point. This can be done, with the help of cylindrical lenses.

Cylindrical lenses monitor the way the eye focuses the light, and enables equal refraction of light at every point and aid in providing a clear vision.

Hence, option (b) cylindrical lens is the correct option.

Note:Concave lenses are used to correct myopia which is another “refractive error”, where nearby objects are seen clearly, but further objects become blurred. The convex lens can be used to cure hypermetropia, where one cannot see the far away objects clearly. Similarly, presbyopia can be cured by a bifocal lens.