Question

Two lenses have power(i)+2 dioptre,(ii)-4 dioptre.What is the nature and focal length of each lens

Answer 1

(i)  power = + 2 Dioptre
   => f = 1/P = 1/2 meter = 50 cm
   =>  since focal length and power are positive, it is a convex lens.

(ii) power = - 4 Dioptre
   => f = 1/P = -1/4 meter = - 25 cm
   => as focal length and power are negative,  it is a concave lens.