Question

A person cannot see distinctly objects kept beyond 2m. This defect can be corrected by using lens of power

• A. 0.5 D
• B. -0.5 D
• C. 0.2 D
• D. -0.2 D

Answer 1

The correct option is B

-0.5 D

Step 1: Given Data

1. As per sign convention:
2. The normal position of the far object is $D=-\infty$
3. The position till the far object is seen is $d=-2cm$

Step 2: Calculation of power of the lens

Apply the lens formula to calculate the focal length of the lens:
$$\begin{gathered} \frac{1}{f}=\frac{1}{d}-\frac{1}{D} \\ =\frac{1}{-2}+\frac{1}{\infty } \\ f=-2cm \end{gathered}$$

Power of the lens is defined as the inverse of the focal length of the lens.

$$\begin{gathered} P=\frac{1}{f} \\ =\frac{1}{-2cm} \\ =-0.5D \end{gathered}$$

Therefore, the power of the lens is -0.5 D.

Hence, option (b) is correct