Question

A concave lens of focal length of 10cm and convex lens of focal length 20cm are placed in contact with each other. find the power of this combination and also calculate focal length of the combination?

Answer 1

A concave lens of focal length of 10cm and convex lens of focal length 20cm are placed in contact with each other. find the power of this combination and also calculate focal length of the combination?.

solution : focal length of concave lens, f = -10cm or, -0.1m

so, power of concave lens, $P_1$ = 1/f = 1/-0.1 = -10D

focal length of convex lens , f' = +20cm or, +0.2 m

so, power of convex lens , $P_2$ = 1/f' = 1/+0.2 = +5D

now lenses are combined.

so, power of combination, P = $P_1+P_2$

or, P = -10D + 5D = -5D

hence, power of combination is -5D (diverging)

focal length of combination = 1/P

= 1/-5 = -0.2m = -20cm

hence, focal length of combination is -20cm.