Question

A student focused the image of a candle flame on a white screen by placing the flame at various
distances from the convex lens. He noted his observation in the following table.
SI
NO:
Distance of the
flame
from the lens(cm)
Distance of the
screen
from the lens(cm)
i 20 60
ii 24 40
iii 30 30
iv 40 24
v 70 12
Analyse the above table and give the answers for the following questions:
(a) What is the focal length convex lens?
(b) Which set of observation is incorrect and why?
(c) Draw ray diagram for observation (iv)
(d) Find the magnification for observation (iii).

Answer 1

Answer:

you can see, case (ii) ; here both the

object and the image distances are

30cm .but we know this is possible only

case when object kept at 2f,so, 2f =

30cm >f= 15cm

(b) use lens formula for finding incorrect

observations.

1v-1/u = 1/f

see last observation (v)

u =-15

f +15

then, 1/v = 1/15 - 1/15 = 0 = 1/co =>V = o

but v 70cm is given hence, observation

(v) is wrong.

(c) obviously case 3, in this case size

of image is equal to size of object due

to image distance distance is equal to

object distance from pole.