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Q: The power of a convex lens of focal length 40 cm is _______.
  • A. –2.5 D
  • B. 0.25 D
  • C. 2.5 D
  • D. –0.25 D
Correct Answer: Option C - The power of a lens is defined as the reciprocal of its focal length in meters. D=1/f, where D is the power in diopters and f is the focal length in meters and D= 100/f where D is the power in diopters and f is the focal length in centimeters. Thus, D = 100/40 = 2.5 D
C. The power of a lens is defined as the reciprocal of its focal length in meters. D=1/f, where D is the power in diopters and f is the focal length in meters and D= 100/f where D is the power in diopters and f is the focal length in centimeters. Thus, D = 100/40 = 2.5 D

Explanations:

The power of a lens is defined as the reciprocal of its focal length in meters. D=1/f, where D is the power in diopters and f is the focal length in meters and D= 100/f where D is the power in diopters and f is the focal length in centimeters. Thus, D = 100/40 = 2.5 D