Question

The image of a distant objects is formed at 20 cm from a concave mirror. what is the focal length of the mirror.
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Answer 1

Explanation:

1/v + 1/v = 1/f.

If u = -∞ then v = f

So given v = -20 cm. So f = -20 cm.

A point Image is formed at the focus.

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Convex mirror:

1/u + 1/v = 1/f

given u = -∞

1/v = 1/f

of v= f. f is positive. Image is behind the mirror.

When object is at infinity, then a point image (erect) at the focus is formed.

Answer 2

The distance between the pole P of the concave mirror and the focus F is the focal length of the concave mirror. Thus, the focal length of a concave mirror can be estimated by obtaining a 'Real image' of a distant object at its focus, as shown in the figure.