Question

The power of a lens is $–2D$. What is its focal length?

• A. $-50cm$
• B. $50cm$
• C. $5cm$
• D. $-5cm$

Answer 1

The correct option is A

$-50cm$

Explanation

Focal length: The focal length of an optical system is the inverse of the system's optical power.

1. It measures how strongly the system converges or diverges light.
2. A system with a positive focal length converges light, while one with a negative focal length diverges light.

Step 1: Given

Power of the lens= $-2D$

Step 2: calculating focal length

As we know,

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

Where,

D is Power of the lens in diopters

f is focal length of the lens in meters

So, substituting the known values,

$$\begin{gathered} \Rightarrow -2=1/f \\ \Rightarrow f=-\frac{1}{2}m \\ \Rightarrow f=-0.5m \\ \Rightarrow f=-0.5\times 100cm \\ \Rightarrow f=-50cm \end{gathered}$$

Hence, option A is the correct answer.