Define power of a lens. What is its unit? One student uses a lens of focallength 50 cm and another of –50 cm. What is the nature of the lens and itspower used by each of them?

The power of a lens is defined as the reciprocal of its focal length. It is a measure of how much the lens converges or diverges light.

The unit of power of a lens is called Dioptre (D). It is defined as 1 meter to the power -1.

For the first student, the focal length of the lens is 50 cm or 0.5 m. Therefore, the power of the lens is 1/0.5 = 2 D. Since the power is positive, the lens is a converging or a convex lens.

For the second student, the focal length of the lens is -50 cm or -0.5 m. Therefore, the power of the lens is 1/-0.5 = -2 D. Since the power is negative, the lens is a diverging or a concave lens.