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Question
How far should an object be placed from the pole of a converging mirror of focal length 20cm to form a real image of the size exactly `1/4`th the size of the object?
Solution
Given,
Focal length of the concave mirror (f) = 20 cm
Magnification (m) = -1/4
Using the magnification formula, we get
`m=-v/u`
`-1/4=(-v)/u`
`v=u/4`
Using the mirror formula, we get
`1/f=1/v+1/u`
`1/-20=4/u+1/u`
`-1/20=5/u`
u=−100 cmThus, the object should be placed in front of the concave mirror at a distance of 100 cm.
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