Question

A person is not able to see the object at the distance more than 40 cm from his eye. In order to see the object clearly, he must use the lens of power

• A. –40 D
• B. +40 D
• C. –2.5 D
• D. +2.5 D

Answer 1

The correct option is C –2.5 D

As the person can see the object up to 40 cm only, the image of a distant object should be formed by the lens at a distance of 40 cm to see it clearly.
For distant object, object distance, u = – ∞
Image distance, v = –40 cm = –0.40 m
Using lens formula, $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$P=\frac{1}{-0.40}-\frac{1}{(–\infty )}$
As power, $P=\frac{1}{f}$
$P=-2.5D$
Hence, the correct answer is option (c).