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The distance between object and its 3 times magnified virtual image as produced by a convex lens is 20 cm. The focal length of the lens used is ______ cm.

Asked in JEE Main 1st Feb 1st Shift 2024 · Lens formula and magnification

Answer: 15

Step-by-step solution

Virtual image is on the same side as the object: |v| = 3|u|.

Separation |v| - |u| = 2|u| = 20, so u = -10 cm, v = -30 cm.

1/f = 1/v - 1/u = -1/(30) + 1/(10) = 1/(15)

f = 15 cm

→ 15

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