Question

If the power of a lens is $+5$ dioptre, what is its focal length?

Answer 1

Step 1 - Given data.

Power of the lens is, $P=+5D$

Step 2 - Finding the focal length.

The relation between the focal length and the power of the lens is given by, $P=\frac{1}{f}$ where $f$ is the focal length.

$$\begin{gathered} +5D=\frac{1}{f} \\ f=0.2m\left(1m=100cm\right) \\ f=20cm \end{gathered}$$

Hence, the focal length of the lens is $+20cm$.