Question

You use a concave lens of focal length $2m$. What is the power of the lens?

Answer 1

Step 1: Given data

Focal length, $f=2m$

Step 2: Formula

Expression for the power of the lens.

Power of lens

The power of the lens is defined as the reciprocal of the focal length.

$P=\frac{1}{f}$

Substitute $2m$for $f$ in above expression

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

Hence the required Power is $-0.5D$. The negative sign is written because it is mentioned that the lens is concave in shape.