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An optician prescribes a power = - 0.5 dioptre. The corresponding lens must be a

  1. convex lens of focal length 2 m
  2. concave lens of focal length 50 cm
  3. concave lens of focal length 2 m
  4. convex lens of focal length 50 cm

Answer

P = 1/f

f = 1/0.5 = 2 m

The power of a convex lens is positive and concave lens is negative.

The correct option is C.