Question

A student focussed the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle, screen and the lens with metre scale as under:
Position of candle $=12.0cm$
Position of convex lens $=50.0cm$
Position of the screen $=88.0cm$

Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm ?

• A. at infinity
• B. at position of screen.
• C. at position of convex lens
• D. at position of candle.

Answer 1

The correct option is A at infinity

Before shifting the candle,

$u=12-50=-38cm$

$v=88-50=38cm$

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

$\frac{1}{f}=\frac{1}{38}+\frac{1}{38}$

$\frac{1}{f}=\frac{1+1}{38}$
$f=19cm$

After shifting the candle,

$u=31-50=-19cm$

Hence the object is at focus of convex lens .

So, the image will be formed at infinity.