Question

two thin lenses of focal lengths + 20 cm and -15 CM are kept in contact what is the focal length and power of the combination

Answer 1

First focal length is +20 cm

Therefore, power =1/20cm

=1/0.2 m

=+5D

Again, F2 is -15 cm

Therefore, power =1/-15 cm

=1/-0.15 m

=-6.6D

Thus, power = 5-6.6

= -1. 6 D

Now, we know that,

F =1/P

=1/-1.6

=-0.625 m

Answer 2

Answer:refer the attachment

Explanation: