Question

What is focal length when concave lens and convex lens are joined?

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..