Question

A luminous object and a screen are placed on an optical
bench and a converging lens is placed between them
to throw a sharp image of the object on the screen, the
linear magnification of the image is found to be 2.5.
The lens is now moved 30 cm nearer the screen and a
sharp image is again formed. Calculate the focal length
of the lens

(a) 12.3 cm
(c) 14.3 cm
(b) 13.3 cm
(d) 15.3 cm​

Answer 1

14 .3 cm

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Answer 2

Answer:

c) 14.3

Explanation:

m=2.5

u=x

v/u=2.5

v=2.5x..........(i)

v=30+x............(ii)

put in equation (i) in (ii)

2.5x=30+x

30=2.5x-x

30=1.5x

x=30/1.5 =20cm

:. u=20cm

from equation (ii)

v=30+20

=50cm

1/f=1/v - 1/u

1/f=1/50-1/20 =2+5/100=7/100

f=100/7

=14.3cm