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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 : (a) the focal length of the mirror (b) position of image
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Solution
m = `"I"/"O" = 20/"O" = 2`
∵ m = `-"v"/"u"`
∴ `-"v"/"u" = 2`
∴ -v = 2u
∵ u = -5 cm
∴ v = 10 cm
Hence , the position of the image is 10 cm behind the mirror.
From 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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