Question

Why focal length of concave lens is negative & of convex it is positive?

Answer 1

There are some conventions when we measure distance in the opposite direction of the incident ray it is taken to be negative else positive
In concave lens the light rays get diverged and these rays meet at a point in the same side of the incident ray (. i.e.)in direction opposite to incident ray on principal axis hence its focal length is negative
While in convex lens the light rays get converged at a point on principal axis on the opposite side of incident ray (i.i.) along the direction of incident ray hence its focal length is negative


thus we say that
focal length of concave lens is negative & of convex lens it is positive


Hope this helped u :-)

Answer 2

Answer:

refer to the above attachment

hope it helps uh :-)