Question

A boy uses spectacles of focal length$-50cm$. Name the defect of vision he is suffering from and also calculate the power of this lens.

Answer 1

Step 1: Defect of vision

The focal length of the spectacles is $f=-50cm$.
As the focal length of the lens is negative, the lens is concave.
A concave lens is used to correct myopia.
Therefore, a boy is suffering from myopia or nearsightedness.

Step 2: Finding the power of the lens

According to the formula for the power of the lens,

$P=\frac{100}{f\left(cm\right)}D$

where,
$P$ is the power of the lens
$f$ is the focal length of the lens
$D$ is the unit of power of lens measured in dioptre

Substituting known values

$$\begin{gathered} P=\frac{100}{\left(-50cm\right)}D \\ P=\frac{100}{-50}D \\ P=-2D \end{gathered}$$

Therefore, the power of the lens is $-2D$.