Question

Let spherical aberration and chromatic aberration be denoted by S and C respectively. In case of concave mirror,

• A. S and C both may be present
• B. Both S and C can not exist
• C. S may be present but C cannot exist
• D. C may be present but S cannot exist

Answer 1

Answer: (C)

S may be present but C cannot exist

Figure A shows spherical aberration for a concave mirror for an object at infinity.The rays parallel to the principal axis are incident on the spherical surface of the concave mirror. The rays close to the principal axis (paraxial rays) are focused at the geometrical focus of the mirror. The rays farthest from the principal axis are called the marginal rays and are focused at a point F somewhat closer to the mirror. Hence S may be present.
In figure B, The violet rays are deviated more and hence, they form an image closer to the lens as compared to the image formed by the red rays. If light is incident on the lens from left to right, the violet image is to the left of the red image for convex lens and it is to the right of the red image for the concave lens. In the first case, the chromatic aberration is called positive and in the second case, it is negative. Thus, a proper combination of a convex and a concave lens may result in no chromatic aberration. Hence C cannot exist