Physicsmedium · Past Paper
What is the focal length of a lens with power -4.0 D?
A+25 cm
B-25 cm
C-40 cm
D+40 cm
✓ Correct Answer: B — -25 cm
f = 1/P = 1/(-4) = -0.25 m = -25 cm. The negative sign indicates a concave lens.
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