Question

How can the spherical aberration produced by a lens be minimised?

Answer 1

Hint: We have noticed that sometimes a defect occurs in the spherical lens. When parallel light rays strike the lens' surface, instead of converging at a common point, the rays converge at a different point on the optical line axis.

Complete step by step solution:
When the rays are converging at different points, we can converge a blurry image due to it. It happens due to inconsistency in the curvature of the lens or when the lens' curvature profile is not perfect such that the rays at the outer edge of the lens converge more than the rays that are close to the centre point of the lens.
We will now see the following methods can eliminate that spherical aberration:
We can separate the rays by blocking them so that either the marginal rays are blocked, or the paraxial rays are blocked. We can eliminate the rays that are close to the principal axis, or we can either eliminate the rays that are away from the principal axis with opaque screens.
One more way to eliminate this effect is to further modify the lens by using a non-spherical profile either at the centre part of the lens or at the lens's periphery.

Therefore, the spherical aberration produced by a lens can be minimised by blocking paraxial rays and marginal rays.

Note:
There are many combinations of lens profiles to eliminate the effect of aberration. We should keep in mind that when a conic section profile is used in combination with a plane surface lens, this spherical aberration is significantly reduced.