A spherical mirror having focal length f=−25cm 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=+25cm
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B
u=−30cm
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C
u=−25cm
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D
u=−50cm
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Solution
The correct option is Du=−50cm Given that, f=−25cm
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=−50cm ∴u=−50cm
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