Question

Calculate the focal length of a lens, if its power is $–2.5D$.

Answer 1

Step 1: Given parameters

The power of the lens, $P=–2.5D$

Step 2: Formula used

The power of the lens can be expressed as,

$$\begin{gathered} P=\frac{1}{f} \\ f=\frac{1}{P}...\left(1\right) \end{gathered}$$

Where, $P$ is the power of the lens, and $f$ is the focal length of the lens.

Step 3: Calculation of focal length

Substitute the values in equation (1).

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

$$\begin{gathered} f=\frac{1}{-\left(2.5\right)} \\ f=-\frac{10}{25} \\ f=-0.4\mathrm{m} \\ f=-0.4\times 100\mathrm{cm}=-40cm\left(\because 1\mathrm{m}=100\mathrm{cm}\right) \end{gathered}$$

Step 4: Conclusion

Therefore, the focal length of the lens is $-40\mathrm{cm}$.