Question

Define the following terms: (i) Power of a lens (ii) Principal focus of a concave mirror

Answer 1

(i) Power of a lens:

1. It is the ability of a lens to converge or diverge light rays falling on it. It is given by the reciprocal of the focal length of the lens.
2. $i.e.,poweroflens=\frac{1}{focallength}orP=\frac{1}{f}$
3. If the focal length is given in meters (m), the power of the lens is measured in Diopter (D), as in the unit of power of the lens is diopter.
4. For a converging lens the optical power is positive and for a diverging lens, it is negative.
5. The shorter the focal length, the larger the power and vice-versa.

(ii) Principal focus:

1. The principal focus of a concave mirror is a point on the principal axis at which a light ray parallel to the principal axis converges after reflection.
2. Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.
3. It is denoted by F.