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JEE Main Physics Class 12 Ray Optics and Optical Instruments Part 1 Questions
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Question : 15
Total: 99
A thin lens made of glass (refractive index = 1.5) of focal length f = 16 cm is immersed in a liquid of refractive index 1.42. If its focal length in liquid is
f
l
,then the ratio
f
l
∕
f
is closest to the integer:
[7 Jan. 2020 II]
1
9
5
17
Validate
Solution:
Using lens maker's formula
1
f
=
(
µ
g
µ
a
−
1
)
[
1
R
1
−
1
R
2
]
Here,
µ
g
and
µ
a
are the refractive index of glass and air respectively
⇒
1
f
=
(
1.5
−
1
)
(
1
R
1
−
1
R
2
)
.......(i)
When immersed in liquid
1
f
l
=
(
µ
g
µ
l
−
1
)
(
1
R
1
−
1
R
2
)
[Here,
µ
l
=
refractive index of liquid]
⇒
1
f
l
=
(
1.5
1.42
−
1
)
(
1
R
1
−
1
R
2
)
.......(ii)
Dividing (i) by(ii)
⇒
f
l
f
=
(
1.5
−
1
)
1.42
0.08
=
1.42
0.16
=
142
16
≈
9
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