Physicshard · Past Paper
If a convex lens of focal length 20 cm and a concave lens of focal length 50 cm are combined, the system behaves as a:
AConvex lens, f = 33.3 cm
BConcave lens, f = 33.3 cm
CConvex lens, f = 70 cm
DConcave lens, f = 70 cm
✓ Correct Answer: A — Convex lens, f = 33.3 cm
1/F = 1/20 - 1/50 = (5-2)/100 = 3/100. F = 33.3 cm. Positive sign means convex.
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