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A myopic person has a power of -1.25 Dioptre. What is the focal length and nature of his lens?

  1. 50 cm and convex lens
  2. 80 cm and convex lens
  3. 50 cm and concave lens
  4. 80 cm and concave lens


P = 1/f

f = 1/P = 1/1.25 = 0.8 m or 80 cm

Convex lenses, or converging lenses, have positive values, as they magnify objects. A diverging lens or concave lens, on the other hand, have a negative dioptre, since it makes objects appear smaller than they are.

The correct option is D.