Question

Select One Correct Option from the following: An eye specialist prescribes spectacles having combination of convex lens of focal length 40 cm in contact with a concave lens of focal length 25 cm. The power of thin lens combination in diopters is
(A) +1.5
(B) −1.5
(C) +6.67
(D) −6.67.

Answer 1

Ans.(B) -1.5

focal length of convex lens f1=40cm=0.4m

power of convex lensP1 =1/f1=5/2=2.5D

focal of concave length f2 =25cm=0.25m

power of concave lensP2 =1/f2=-4D

power of combination=P1 +P2=2.5 -4=-1.5D