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Question
A concave mirror forms a virtual image of size twice that of the object placed at a distance 5 cm from it. Find:
- the focal length of the mirror
- position of image
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Solution
Given: Object distance, u = −5 cm (object is in front of the mirror)
The image is virtual and erect
Magnification, m = +2m
(a) m = `-v/u`
∴ `-v/u = 2`
∴ −v = 2u
∴ −v = 2 × −5
∴ v = 10 cm
Hence, the image is 10 cm behind the mirror.
(b) From the mirror formula,
`1/f = 1/v + 1/u`
∴ `1/f = 1/10 + 1/(-5)`
= `1/10 - 1/5`
∴ f = −10 cm
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