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 = -24cm

 v = 74-50=24cm

lens formula= 1/f=1/v-1/u

                   1/f= 1/24- (-1/24)

                    1/f= 1/24 +1/24 =  2/24 = 1/14

                     f = 14cm

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.