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Question

An optician prescribes spectacles to a patient with a combination of a convex lens of focal length 40 cm and a concave lens of focal length 25 cm. What will be the power of spectacles?

  1. 1.5 D
  2. 1.5 D
  3. -2.5 D
  4. -1.5 D


Solution

The correct option is D -1.5 D
Focal length of the convex lens (fA) = 40 cm = 0.4 m​

Focal length of the concave lens (fB) = -25 cm = - 0.25 m​
                     
Power of a lens (P)  =  1f
PA = 1fA  = 10.4m = 2.5 D

PB = 1fB  = 10.25m = -4 D

Combined power of spectacles with both the lenses = PA + PB
                          = 2.5 D + (-4 D) ​
                          = - 1.5 D​

​Power of spectacles having both the given lenses is -1.5 D​

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