Question

The focal length of the lens of a myopic person is $40cm$. What is the power of the lens?

Answer 1

Step 1: Given data

Focal length of the lens for a myopic or short-sighted person is, $f=40cm$.

Step 2: Finding the power ($P$) of the lens

Since the person is myopic, it is a Concave lens.

So, the focal length will be negative.

Applying the formula and substituting the known value,

$$\begin{gathered} \mathrm{P}=\frac{100}{f\left(\text{in}\mathrm{cm}\right)} \\ P=\frac{100}{-40} \\ P=-2.5D \end{gathered}$$

Hence, the power of the lens is $-2.5D$.