The magnification of an object placed in front of a convex lens of focal length 20 cm is +2. To obtain a magnification of −2, the object has to be moved by a distance equal to:
A
10 cm
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B
30 cm
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C
20 cm
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D
40 cm
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Solution
The correct option is C20 cm From the formula of magnification, m=vu=2 v=2u
Using lens formula, 1v−1u=1f 12u−1u=1+20 u=−10 cm
New magnification, m′=v′u′=−2 v′=−2u′
Again, using lens formula, 1v′−1u′=1f 1−2u′−1u′=1+20 u′=−30 cm
Distance by which object moved, d=|u′−u| d=|−30−(−10)| d=20 cm