Lens power in diopters
A converging lens has a focal length of 25 cm. What is its power?
Answer C: explanation for every choice
A
This uses 25 m instead of 25 cm.
B
This writes the focal length in meters (0.25) instead of taking its reciprocal.
C
Correct. P = 1/f in meters = 1/0.25 = +4 D.
D
This uses the focal length in centimeters as the power.
TakeawayPower (diopters) = 1/f (meters). Converging lenses are positive.
Equation usedLens power
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