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Standard X
Physics
Question

A light source is placed 100cm away from a screen. A converging lens placed at a certain position between the source and the screen focuses the image of the source on the screen. The lens is moved a distance of 40cm and it is found that it again focuses the image of the source on the screen. The focal length of the lens is
  1. 40cm
  2. 30cm
  3. 21cm
  4. 67cm

A
40cm
B
67cm
C
30cm
D
21cm
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Solution
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Answer is A.

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