Define power of a lens and write its SI unit

Define power of a lens and write its SI unit. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the lens, if image size is equal to the object size? Also, find the power of the lens.

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