GK QUESTION#2916
What will be the focal length of a convex lens with the power of a +2.5 Diopter?
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Answer
C. 40 cm
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Explanation
Lens power in dioptres is P = 1/f when f is in metres. Thus f = 1/2.5 = +0.40 m = +40 cm. The positive sign identifies a converging lens in the usual convention.