Question

Answer the following question.

What is focal power of a spherical mirror or a lens? What may be the reason for using P = `1/"f"` as its expression?

Answer 1

1. Converging or diverging the ability of a lens or of a mirror is defined as its focal power.
2. This implies, the more the power of any spherical mirror or a lens, the more is its ability to converge or diverge the light that passes through it.
3. In the case of a convex lens or concave mirror, the more the convergence, the shorter is the focal length as shown in the figure.
   
4. Similarly, in the case of a concave lens or convex mirror, the more the divergence, the shorter is the focal length.
5. This explains that the focal power of any spherical lens or mirror is inversely proportional to the focal length.
6. Hence, the expression of focal power is given by the formula, P = `1/"f"`.