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. Given v = 50 cm, since the size of image and that of the object is same, therefore, the image is formed at 2f.
Hence, 2f = 50 or f = 25 cm = 0.25 m
Now, P = 1/ f = 1/ 0.25= 100/ 25= +4 D