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 = 26-50

= - 24cm

v = 74-50

= 24cm

So from the above data focal length of the lens is 12cm (from the formula 1/f=1/v-1/u)

If the position of the candle is shifted to 38cm  then the object position is at

F1 then the image is formed at infinity (relevant ray diagram to be drawn) 

1/v=1/f+1/u  (u=20-38 =12cm)

Answer 2

Answer:

( i ) .  Object distance u = 50 - 26 = 24 cm

Image distance v = 74 - 50 = 24 cm .

Since object distance is equal to image distance.

2 f = 24

= > f = 12 cm

So , focal length of object is 12 cm.

( ii ) . u = 50 - 38 = 12 cm

It's mean candle is at focus.

= > Image will form at infinity.

( ii ) Refer to attachment for ray diagram.