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An object is kept at a distance of 60 cm...

An object is kept at a distance of 60 cm from a concave mirror. For getting a magnification of `1/2`, focal length of the concave mirror required is

A

20 cm

B

40 cm

C

`-`20 cm

D

30 cm

Text Solution

Verified by Experts

The correct Answer is:
C
`u = - 60 cm, m = (-v)/(u) = (1)/(2)`
`rArr " " v = - 30 `cm (image is real )
`(1)/(v) + (1)/(u) = (1)/(f) `
`rArr " " (1)/(f) = - (1)/(30) - (1)/(60) = - (1)/(20) rArr f = - 20 ` cm.
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