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? (3)

Answer 1

distance of the image from the lens u=26-50 =-24cm
distance of the image from the lensv=74-50  24cm
to calculate the focal length the formula is
1/f=1/v-1/u
1/f=1/24+1/24
1/f=2/24 f=12cm

 so from the above data focal length of the lens is 12cm
if the position of the candle is shifted to 38cm  then the object position is at F1 then the image is formed at infinity (1/v=1/f+1/u  (u=20-38 =12cm)

Answer 2

i) 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.