Question

a concave lens of focal length 20 cm is placed in contact with a convex lens of same focal length so find the focal length of combination

Answer 1

Here we have a combination of concave and convex lens both have a focal length of 10 cm .
Acc. to sign convention:-
Convex lens has focal length(f1) = + 10cm
Whereas,
Concave lens has a focal length(f2) = -10cm
So focal length by combination are :-
(I/net focal length) = (1/f1)+(1/f2)
After putting the value
(1/net focal length) =0
Thus, net focal length= 0/1 i. e. infinite
Thus it will behave as a plane mirror with focal length infinity.
Thnx.