Question

define focal length of concave and convex lens​

Answer 1

hey                                                                                                                       Focal length is distance over which the parallel rays either converge or diverge. For convex lens the focal length is positive. On the other hand, the focal length of concave lens is negative. Thus, for convex lens, the focal length is a distance over which all the parallel rays will converge.

Hope I helped.

Thanks :)