Question

Explain spherical aberration for spherical
mirrors. How can it be minimized ?
State the restrictions for producing appreciably
clear images produced by spherical mirrors.​

Answer 1

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A spherical mirror forms reasonably sharp image if its aperture is small compared with its focal length. But if the mirror size is large, the peripheral or abaxial rays reflected from the outer edges are brought to a focus closer to the pole than the paraxial rays. This focusing defect is called spherical aberration. The distance between the paraxial and abaxial foci is called the longitudinal spherical aberration (LSA).

Spherical aberration can be reduced by allowing only paraxial rays. This can be achieved by

(1) using a mirror of small size

(2) using a circular iris diaphragm, called an aperture.

With any of these two restrictions, sharp images of objects at any distance greater than the focal length may be formed on a screen, since bundles of parallel rays close to the axis and making only small angles with it are brought to a sharp focus in the focal plane.

If, however, the light is not confined to the paraxial region, parallel incident rays at increasing distances from the principal axis cross the axis closer to the mirror, If a small screen is placed at the paraxial focal plane F and then moved toward the mirror, a point is reached where the size of the circular image spot is a minimum. This circular spot is indicated and is called the circle of least confusion.