Question

Focal length of a concave mirror is 20cm. At what distance an object should be placed infront of it so that its image may be obtained at a distance of 12 cm

Answer 1

Explanation:

v=-12cm

f=-20cm

By mirror formula,

1/v+1/u=1/f

1/-12+1/u=1/-20

1/u=1/-20+1/12

1/u=-3+5/60

1/u=2/60

u=30cm

Therefore the object distance will be 30cm

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Answer 2

Answer: - 7.5 cm

Explanation:

Focal length of a concave mirror= -20 cm

Image distance = 12cm

Using the Mirror formula,

I/f= 1/u + 1/v

=> -1/20 = 1/u + 1/ 12

=> 1/u = -1/20 -1/12

= -8/60

=> u = -60/8 cm

= - 7.5 cm