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Question

A spherical mirror having focal length f=25 cm is used to obtain an image which is of the same size as that of the object. What should be the object distance (u) as per sign convention?

A
u=+25 cm
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B
u=30 cm
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C
u=25 cm
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D
u=50 cm
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Solution

The correct option is D u=50 cm
Given that, f=25 cm
Focal length of concave mirror is always negative, because focal length is measured opposite to the direction of incident ray.

In the case of a concave mirror, same size of image is obtained when the object placed at the centre of curvature.
so, u=R=2f=50 cm
u=50 cm

Concept involved:-
(1) Focal length of a concave mirror is always negative.
(2) When an object placed at centre of curvature,
Image height = Object height

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