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Question

An object is 20 cm away from a stationary concave mirror having focal length 15 cm. If the object moves with a speed of 5 m/s along the principal axis towards the mirror, then the speed of the image will be

A
45 m/s
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B
40 m/s
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C
35 m/s
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D
30 m/s
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Solution

The correct option is A 45 m/s
For the given concave mirror,
u=20 cm, f=15 cm
Using mirror formula,
1v120=115
1v=160
v=60 cm
So, m=vu=(60)20=3
Also, velocity of the image when the object is moving along the principal axis,
vi,m=m2vo,m
vi,m=(3)2×5=45 m/s
Hence, the speed of the image will be 45 m/s.
Why this question?Caution: While applying formula of magnification and image velocity, alwaystake care of sign convention.

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