Question

Rohit focused the image of a candle flame on a white screen using a convex lens.
He noted down the position of the candle, screen and lens as under:
Position of candle = $26.0$cm
Position of convex lens = $50.0$ cm
Position of screen = $74.0$ cm

• A. What is the focal length of the convex lens?
• B. Where will the image be formed if he shifts the candle towards the lens at a position of $38$ cm?
• C. Draw a ray diagram to show the formation of the image in case (ii) as said above?
• D. None of the above

Answer 1

The correct option is A What is the focal length of the convex lens?

(A).Distance of object from lens$u=50-26=24cm$

Distance of the screen from the lens$v=74-50=24$

From lens formula,

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

$\frac{1}{24}-\frac{1}{-24}=\frac{1}{f}$

$\frac{24}{2}=-f$

$f=-12cm$

(B). Now when the object is shifted to distance of 38 cm $u=50-38=12cm$

Since $u=f$, the image will form at infinity.