Question

A convex lens of power $+4\text{D}$ is placed at $40\text{cm}$ from a wall. At what distance from the lens should a candle be placed so that its image is formed on the wall?

• A. $\frac{100}{3}\text{cm}$
• B. $\frac{200}{3}\text{cm}$
• C. $\frac{400}{3}\text{cm}$
• D. $\frac{500}{3}\text{cm}$

Answer 1

The correct option is B $\frac{200}{3}\text{cm}$

Given, power, $P=+4\text{D}$.

So, focal length, $f=\frac{1}{P}=\frac{1}{4}\text{m}=+25\text{cm}$

Now,

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\Rightarrow \frac{1}{40}-\frac{1}{u}=\frac{1}{25}$

$\Rightarrow \frac{1}{u}=-\frac{3}{200}$

$\therefore u=-\frac{200}{3}\text{cm}$