Calculate the focal length of a convex lens which forms... – Physics

Physicsmedium · Past Paper

Calculate the focal length of a convex lens which forms a real image at 20 cm when an object is placed 20 cm away.

A20 cm

B10 cm

C40 cm

D5 cm

✓ Correct Answer: B — 10 cm

Using 1/f = 1/v - 1/u: 1/f = 1/20 - 1/(-20) = 1/20 + 1/20 = 2/20 = 1/10. So, f = 10 cm.

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