The same size images are 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.

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answer is 15.

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Detailed Solution

We know for Lens, m=ff+u Case 1: −m=ff+u1 Case 2:+m=ff+u2 Equating both ⇒−ff+u1=ff+u2⇒−f+u1=f+u2⇒−(f−20)=(f−10)⇒−f+20=f−10⇒2f=30⇒f=302=15cm

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