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A lens has a power of +2.5 D. What is its focal length?

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Question: A lens has a power of +2.5 D. What is its focal length?

Options:

  1. 40 cm
  2. 25 cm
  3. 50 cm
  4. 20 cm

Correct Answer: 25 cm

Solution: 

Power (P) = 1/f (in meters). Therefore, f = 1/2.5 = 0.4 m = 40 cm.

A lens has a power of +2.5 D. What is its focal length?

Practice Questions

Q1
A lens has a power of +2.5 D. What is its focal length?
  1. 40 cm
  2. 25 cm
  3. 50 cm
  4. 20 cm

Questions & Step-by-Step Solutions

A lens has a power of +2.5 D. What is its focal length?
  • Step 1: Understand that the power of a lens (P) is related to its focal length (f) by the formula P = 1/f.
  • Step 2: Rearrange the formula to find the focal length: f = 1/P.
  • Step 3: Substitute the given power into the formula: f = 1/2.5.
  • Step 4: Calculate the focal length: f = 0.4 meters.
  • Step 5: Convert the focal length from meters to centimeters: 0.4 meters = 40 centimeters.
  • Lens Power and Focal Length – The power of a lens is the reciprocal of its focal length, measured in diopters (D), where P = 1/f.
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