It is defined as the reciprocal of focal length or it is the ability of a lens to bend the rays of light falling on it. Its SI unit is dioptre.
Object distance, u= - 50 cm
Image distance, v= 50 cm
Focal length = f
According to the lens formula,
u = -50 cm v = + 50 cm
\(\frac{1}{50} + \frac{1}{50} f = + 25 cm\)
= 0.25 m
Power of lens (P) = \(\frac{1}{f} = \frac{1}{ 0.25} = + 4D.\)