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Question : 43
Total: 60
3. A converging lens forms a three times magnified image of an object, which can be take on a screen. If the focal length of the lens is 30 cm
Solution:
Explanation: We know f = R ∕ 2
m = v ∕ u ( m = magnification,
v = image distance, u = object distance)
− 3 u = v
1 ∕ v − 1 ∕ u = 1 ∕ f
− 1 ∕ 3 u − 1 ∕ u = 1 ∕ 30
− u − 3 u ∕ ( 3 u ) = 1 ∕ 30
− 4 u ∕ 3 u 2 = 1 ∕ 30
− 3 u ∕ 4 = 30
u = − 40 cm
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