The image of an object formed by a convex lens is of the same size as the object. If the image is formed at a distance of 40 cm. Find the power of the lens.

A same–sized image is formed when an object is placed at a distance of 2f from a convex lens.

The image is formed at a distance of 2f from the lens.

Here this distance is given as 40 cm.

So, 2f = 40 cm or f = 20 cm

$\text{ Power, }\mathrm{P}=\frac{1}{\mathrm{f}}=\frac{1}{0.2 \mathrm{m}}=5\mathrm{D}$