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.

Hence, 2f = 50 or f = 25 cm = 0.25 m

Now, P = 1/ f = 1/ 0.25= 100/ 25= +4 D