Question

how the focal length of a lens may be determined by the given power of the lens?

Answer 1

as we know ,
focal length is inversely proportional to power of lens.
power of lens is the degree of convergence or divergence of light rays that can be achieved by a lens in terms of power.
power of lens = 1 / focal length (in metres)
power of lens = 100 / focal length (in cm)

hope it helps

Answer 2

power of a lens is equal to reciprocal of its focal length.

power of a lens =1/f(in m) and 100/f(in cm)

example: if given power of lens is 2D (here D refers to dioptres, theunits for power of a lens) then

                using p=100/f

                           2=100/f

                               f=50

                          therefore f is determined