The power of a lens is 4 Diopters. Find the focal length of the lens.

The power of a lens is given by : $\mathrm{P} = \frac{1}{\mathrm{f}}{\mathrm{m}}^{-1}$
Where f is the focal length of the lens.

$\text{P}=\frac{100}{\text{f}} {\mathrm{cm}}^{-1}\Rightarrow 4=\frac{100}{\text{f}}$ 

$\Rightarrow \text{f}=\frac{100}{4}=25\text{cm}$

Therefore, the focal length of the lens will be 25 cm.