Question

A person needs a lens of power $-4\mathrm{D}$ for correcting his distant vision. What is the focal length of the lens required for correcting distant vision?

Answer 1

Step 1: Given data

For distant vision,

Power of lens $=-4\mathrm{D}$

Step 2: Formula used

We know, the power of a lens is given by,

$P=\frac{1}{f(\mathrm{in}\mathrm{meters})}$

Step 3: Finding the focal length

Substitute the value of $P$ as $-4\mathrm{D}$ in the above formula.

$P=\frac{1}{f(\mathrm{in}\mathrm{meters})}$

$$\begin{gathered} -4\mathrm{D}=\frac{1}{f} \\ f=-\frac{1}{4\mathrm{D}} \\ f=-0.25\mathrm{m} \\ f=-25\mathrm{cm} \end{gathered}$$

Hence, the focal length of the lens for correcting distant vision is $25\mathrm{cm}$. Since the focal length is negative it is a concave lens (diverging lens).