Question

calculate the focal length of a corrective lens having power +2.5D

Answer 1

So,
P=1/F
F=1/P,
so we get,
F=1/2.5
F=0.4

Answer 2

To calculate the focal length of any lens , including the spectacles lens,we use a simple mathematical formula.

Formula to calculate the focal length of a lens:

Power of the lens = 1/focal length of the lens

Power=1/focal length

2.5=1/f (as given in the question)

f=0.4

So the approximate focal length of a +2.5D powered lens is almost 0.4m.