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A convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together. What will be their resulting power?

1. +6.5 D
2. -6.5 D
3. +7.5 D
4. -0.75 D

Correct answer: -0.75 D

Solution

The power of a lens is given by P = 100/f (in cm). For the convex lens, P1 = 100/80 = +1.25 D. For the concave lens, P2 = 100/(-50) = -2.0 D. The combined power is P = P1 + P2 = +1.25 - 2.0 = -0.75 D.

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