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

i) What is the focal length of the convex lens?

ii) Where will the image be formed if he shifts the candle towards the lens at a position of 38 cm?

iii) Draw a ray diagram to show the formation of the image in case (ii) as said above?

Answer 1

u = 50 - 26 = 24 cm

v = 74 - 50 = 24 cm

2f = 24 cm

f = 24/2 = 12 cm

ii) u = 50 - 38 = 12 cm

i.e. Candle is at f

So, image is formed at infinity.


I hope my answer is wright..

Answer 2

Let the optic centre be the terminal point of 50 cm

U = 50 - 26 = 24 cm
V = 74 - 50 = 24 cm

Since U (object distance) = V ( image distance)  The object must be placed at 2F.

So 2F = 24cm
F= 24/2 = 12 cm

Therefore focal length is 12 cm.

Now candle is shifted to 38 cm 

Therefore U = 50 - 38 = 12 cm

Since the Object is placed at the focus the image will be formed at infinity.

Hope it helps!!
Cheers!!