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Updated on - Oct 31, 2023 07:51 PM IST

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Related Questions

J

Jayita Ekka, Content Team

-Answered on Feb-11-2025

Dear student,

You are asking if the focal length of a lens is 20 cm, what is its power in diopters?

Calculating Lens Power:

  • Power of a lens is defined as the reciprocal of its focal length when expressed in meters
  • The unit of power of a lens is diopter (D)

Formula:

  • Power (P) = 1 / focal length (in meters)

Given:

  • Focal length (f) = 20 cm = 0.2 meters

Calculation:

  • Power (P) = 1 / 0.2 = 5 diopters

Therefore, the power of the lens is +5 diopters

Why is it positive?

  • A positive power indicates a convex lens (converging lens)
  • Convex lenses are thicker at the center and thinner at the edges
  • They converge light rays to a point called the focus

convex lens

Additional Notes:

  • Concave Lens: If the lens were concave (diverging lens), the power would be negative
  • Units: Always ensure that the focal length is converted to meters before calculating the power in diopters

Therefore, if a lens has a focal length of 20 cm, its power is +5 diopters. This means it is a convex lens capable of converging light rays.

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