An object is placed at a distance of 30cm from a concave mirror and its real image is formed at a distance of 30cm from the mirror. The focal length of the mirror is :
A
60cm
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B
45cm
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C
30cm
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D
15cm
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Solution
The correct option is B15cm Given,
Object distance , u=+30cm and image distance v=+30cm
If f be the focal length, 1/f=1/u+1/v=1/30+1/30=1/15 or f=15cm