Question

The power of a concave lens is $-12.5D$. What is the focal length of the lens?

Answer 1

Step 1: Given data

For a concave lens, the power is, $P=-12.5D$.

Step 2: Finding the focal length ($f$) of the lens

Applying the formula and substituting the known value,

$$\begin{gathered} \mathrm{P}=\frac{100}{f\left(\text{in}\mathrm{cm}\right)} \\ -12.5=\frac{100}{f} \\ f=\frac{100}{-12.5} \\ f=-8cm \end{gathered}$$

So, the focal length of the lens is $-8cm$.