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Question
At what distance should an object be placed from a lens of focal length 25 cm to obtain its image on a screen placed on the other side at a distance of 50 cm from the lens? What will be the magnification produced in this case?
Solution
Given that,
Focal length, f = 25 cm
Image distance, v = 50 cm
Form the lens formula,
`1/v-1/u=1/f` (where u = object distance)
`"or", 1/u=1/v-1/f`
`=1/50-1/25`
Therefore,
`1/u=(-1)/50`
Or, u = −50 cm
Magnification, `m=v/u`
`=50/(-50)`
= -1
The object must be placed 50 cm away from the lens, on the other side to produce a magnification of −1.
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