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Question

# An object is placed at 20 cm in front of a concave mirror produces three times magnified real image. What is focal length of the concave mirror?

A
15 cm
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B
6.6 cm
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C
10 cm
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D
7.5 cm
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Solution

## The correct option is A 15 cmGiven : Object distance, u=−20 cmA concave mirror forms a real magnified image only when the image to be formed is inverted i.e. m=−3Magnification m=−vu∴ −3=−v−20 ⟹v=−60 cmUsing mirror formula : 1v+1u=1f∴ 1−60+1−20=1f⟹ f=−604=−15 cmThus focal length of the mirror is 15 cm.

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