Question

a concave lens of focal length 25 cm and a convex lens of focal length 20 CM are placed in contact with each other. what is power of combination. what is the focal length of the combination ?
.
.
.
answer should be (1D,1m)​

Answer 1

Answer:

the focal length of concave lens will always negative and convex lens focal length will be positive

when we put the lenses in contact total power= p1+p2

and reciprocal of total focal length=1/f1 +1/f2

then power

. p=1/f

p1= 1/-0.25

p2= 1/-0.20

total power =1/-0.25 +1/0.20

-4+5=1

the total reciprocal of focal length=1/-0.25+1/0.20

1/f=-4+5=1

then f=1