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The power P of a lens is defined in terms of its focal length f. What is that relationship, and in what unit is P expressed?
AP equals f, measured directly in diopters
BP equals the reciprocal of f, measured in diopters
CP equals f squared, measured directly in metres
DP equals twice f, measured directly in metres
Answer & Solution
Correct answer: B. P equals the reciprocal of f, measured in diopters
1. The power of a lens is the reciprocal of its focal length, with the focal length expressed in metres.
2. This power is expressed in the unit diopters, D, which are reciprocal metres.
3. Squaring f, doubling f, or leaving P equal to f itself do not match this stated reciprocal relationship.
_Source: OpenStax Physics (CC BY 4.0), Ch 16 "Mirrors and Lenses", section 16.3 Lenses_
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