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A lens prescribed by an optician has a power of -2.0 D. What is its focal length, and what type of lens is it?

Question
A lens prescribed by an optician has a power of -2.0 D. What is its focal length, and what type of lens is it?
f = +0.50 m, convex lens
f = -0.50 m, concave lens
f = -2.0 m, concave lens
f = +2.0 m, convex lens
Answer
f = -0.50 m, concave lens
Detailed Explanation
f = 1/P = 1/(-2.0) = -0.50 m; since the focal length is negative, this is a concave (diverging) lens.
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