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I was wondering whether there is a relation between the focal length and aperture of a spherical mirror (concave or convex). I drew a circle on a piece of paper to represent a sphere of which the mirror forms a part. I drew several chords of different lengths on the circle. As the centre of the circle is fixed, hence the focal length of the mirrors formed is same (R/2). But the aperture of the mirrors is different because the length of the chords is different. Does this mean that there is no relation between the focal length and aperture of spherical mirrors?

Mriganka Parasar
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The focal position decreases as the distance of the rays from the optical axis increases. This distance is limited by the aperture. The fact that rays come to a focus at different positions is known as spherical aberration. Limiting rays with an aperture is one way of controlling spherical aberration.

See Spherical aberration in concave mirrors and Does reflected light from a concave mirror pass through a single point?

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sammy gerbil
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