Question

Find the power of lens if focal length of lens is 40 cm

Answer 1

we knw that 
                   power of lens=1/f
so, by putting the value,
                          p=1/.4
                         ~p=10/4
                          ~p=2.5 diopter

Answer 2

Since power=1/focal length(in m)

40cm= 0.4m

Power will be 1/0.4m=2.5D

Hope this helps!!