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Question

Karan cannot see beyond 5 m. What must be the power (in Dioptre) of the corrective lens to be used by him?
  1. - 1
  2. - 0.2
  3. + 0.1
  4. + 1


Solution

The correct option is B - 0.2
Since Karan can only see upto 5 m, he is suffering from myopia. This means that, his far point is smaller than the far point of a normal eye, i.e., ∞. Thus, if we keep an object at the normal far point, u=∞, the corrective lens of focal length f must produce an image at his eye’s far point, v= -5 m. From the lens formula,

1v  1u = 1f

= 1f = 15  1 

P = 1f = 0.2 D
  Thus, Karan must use a concave lens of power -0.2 D.

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