It is intended to focus an image magnified 4 times on a
screen using a concave mirror of focal length 20cm. Where
should the object be placed?
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Since it is 2 times magnified, m = -2
so -v/u = -2
similarly, 1/v + 1/u = 1/f
so 1/v + 1/u = 1/-20
-v/v + -v/u = -v/-20 (multiply throughout by -v)
-1 -2 = v/20
-3 = v/20
v = -60
-2 = -v/u
-2 = 60/u
u = -30
so the object must be placed 30 cm from the mirror
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