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Question

An object is kept in front of a concave mirror of focal length 20 cm. The image is three times the size of the object.Calculate two possible distance of the object from the mirror.

Solution
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It is given that,

Focal length

f=20cm

hi=3ho

The image is three times the size of the object. So there are two cases.

Case 1. For real image

m=vu=3

v=3u

Using mirror’s formula

1f=1v+1u

120=13u+1u

u=803cm

Case 2. For virtual image

m=vu=+3

v=3u

Again using mirror’s formula

120=13u+1u

u=403cm

The possible values of u are 803cmand403cm.

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