Question

A concave lens is kept in contact with the convex lens of focal length 20 cm. The combination works as a converging lens of focal length 100 cm. Calculate the power of concave lens

Answer 1

Power of concave lens will be equal to - 4 D

Answer 2

Explanation:

Focal Length of Convex Lens (f1)= 20 cm = 0.2 m

Power (P1) = 1 / f1

Power (P1) = 1 / f1

P1=1 /0.2 = 5 D

Focal Length of Concave Lens (f2) = ?

Focal Length of Concave Lens (f2) = ?Power (P2) = 1 / f2

Focal length of Combination (f)= 100 cm = 1 m(It acts like converging lens or Convex lens.)

Power Of Combination lens (P) = 1/ f

Power Of Combination lens (P) = 1/ fP = 1 D

Power Of Combination lens (P) = 1/ fP = 1 DATQ, P = P1 + P2

1 = 5 + P2

P2 = - 4 D

Hence , P2=( -4D)