Which eyepiece satisfies the condition of minimum spherical aberration?
A. Huygens’ eyepiece
B. Ramsden’s eyepiece
C. both a and b
D. none

Hint: Huygens’s eye piece is a special two lens arrangement that is capable of avoiding chromatic and the spherical aberrations. Here the lens on which cross wires are made is shifted a bit towards the objective.

Complete answer:
We must know firstly what spherical aberration means. It refers to the phenomenon when light rays passing through a spherical surface fail to focus at a single point. We know that Ramsden’s eyepiece is essentially a Huygens’s eye piece with the bottom lens element reversed. Huygens’s eye piece is a special arrangement of the lens that is able to avoid the issues of the spherical aberration and chromatic aberration. This eyepiece satisfies the condition of reducing the spherical aberration. As we know that Huygens’s eye piece is a superior eye piece because it gives a better image of the object. In this Huygens’s eyepiece, both the condition of achromatism that is minimum chromatic aberration and minimum spherical aberration gets satisfied.
As we come to know that Huygens’s eye piece satisfy the condition of the minimum spherical aberration.

Therefore the correct option for the given question is (a).

Note:
Aberration is the deviation of light rays through lenses, causing images of objects to be blurred. In an ideal system, every point on the object will focus to a point of zero size on the image in case of spherical aberration the lenses are spherical in shape. Sometimes, dust and scratches that are present on the lens would appear in the image in Huygens’s eyepiece this problem does not occur.