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The same size images formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is ______ cm.

Asked in JEE Main 25th Feb 1st Shift 2021 · Lens formula and magnification

Answer: 15

Step-by-step solution

One image is real (object at 20 cm), the other virtual (object at 10 cm), same size.

|m| = |f/(f - x)|: f/(20 - f) = f/(f - 10)

20 - f = f - 10, so f = 15 cm

→ 15

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