Question

find the focal length of bi-concave lens

Answer 1

f =r/2

r is the radius of curvature
f is the focal length

Answer 2

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 lengthis a distance over which all the parallel rays will converge. A certain point over that distance all the parallel rays will converge, that point is known as focalpoint.