22) A luminous object & a screen are placed on an optical bench & a converging lens is placed
between them to through 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 30cm nearer the
screen & a sharp image is again form. Calculate the
focal length of length of the lens.
DIC
- 20 cm
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12th
Physics
Ray Optics and Optical Instruments
Image Formation by Lenses
An object and a screen are ...
PHYSICS
An object and a screen are mounted on an optical bench and a converging lens is placed between them so that a sharp image is received on the screen. The linear magnification of the image is 2.5. The lens is now moved 30 cm nearer to the screen and a sharp image is again formed on the screen. The focal length of the lens is:
A
14.0cm
B
14.3cm
C
14.6cm
D
14.9cm
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VIDEO EXPLANATION

12th
Physics
Ray Optics and Optical Instruments
Image Formation by Lenses
An object and a screen are ...
PHYSICS
An object and a screen are mounted on an optical bench and a converging lens is placed between them so that a sharp image is received on the screen. The linear magnification of the image is 2.5. The lens is now moved 30 cm nearer to the screen and a sharp image is again formed on the screen. The focal length of the lens is:
A
14.0cm
B
14.3cm
C
14.6cm
D
14.9cm
HARD
Share
Study later
VIDEO EXPLANATION

ANSWER
Given uv=2.5
v=2.5u
again v−u=30
v=30+u
2.5u=30+u
1.5u=30
u=20
Now, f1=u1+v1=uvu+v
f=u+vuv=3.5u2.5u2
=75u=75×20=7100=14.3cm
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