Question

The focal length of a convex lens is $50$ cm. What is its power?

Answer 1

Focal length$\left(\mathrm{f}\right)$:

1. It is the measure of the distance between the principle focus and the optical centre of the lens.
2. SI unit of focal length is metre(m).

Power of lens$\left(\mathrm{P}\right)$:

1. It is measured as the reciprocal of focal length when taken in metres. i.e. $\mathrm{P}=\frac{1}{\mathrm{f}}$
2. SI unit of power of lens is Dioptre (D).

Step-$1$: Given data:

The focal length of lens = $50\mathrm{cm}$ $=\frac{50}{100}=0.5\mathrm{m}$

Step-$2$: Calculate power of lens:

$$\begin{gathered} \Rightarrow \mathrm{P}=\frac{1}{\mathrm{f}} \\ \Rightarrow \mathrm{P}=\frac{1}{0.5}=2\mathrm{D} \end{gathered}$$

Hence, the Power of lens of focal length $50$ cm is $2\mathrm{D}$.