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It seems that the reflected ray don’t converge at a point but are distributed in a region. For a concave mirror only those parallel rays which are incident on the mirror near the vertex are focussed at ‘F’, other reflected rays form a fuzzy image.

Thus the failure of rays to converge or diverge from a single point after reflection from a spherical mirror is called spherical aberration. This defects destroy the sharpness of the image.

It can be reduced by:

(a) Using mirrors of small aperture.

(b) Putting a stop in front of marginal rays.

(c) Using parabolic mirror instead of spherical mirrors. [It is for this reason that parabolic mirror do not show spherical aberration. Mirrors used in search lights or head lights in automobiles are parabolic]