Question

what meant by power of lens write it's si unit a student uses a lens of focal length 40cm and another of -40cm write the nature and power of each

Answer 1

It is the reciprocal of the focal length in meter.
Thus, power of a lens=1/focal length
of the lens (in meter)

Answer 2

power of the lens is defined as the reciprocal of focal length in metres
the SI unit of power is diopter positive focal length has converging nature while negative focal length has diverging nature