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Question

An object is placed at a distance of 20 cm from a concave mirror and its real image is formed at distance of 20 cm from the mirror. The focal length of mirror is

A
20 cm
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B
10 cm
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C
15 cm
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D
30 cm
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Solution

The correct option is B 10 cm
For a real image, when object distance = image distance = 20 cm
It means object is placed at centre of curvature of concave mirror.
R=20 cm
2f=20 cm
f=202=10 cm
So the focal length is 10 cm

Why this question?

For spherical mirrors whose aperture is much smaller than its radius of curvature, we use R=2f.

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