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Manipal Entrance Test 2018 Physics Paper
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© examsnet.com
Question : 8
Total: 50
A thin convex lens of refractive index 1.5 has
20
c
m
focal length in air. If the lens is completely immersed in a liquid of refractive index
1.6
,
its focal length will be
- 160 cm
- 100 cm
+ 10 cm
+ 100 cm
Validate
Solution:
From lens formula
1
f
=
(
a
µ
g
−
1
)
(
1
R
1
−
1
R
2
)
=
(
1.5
−
1
)
(
1
R
1
−
1
R
2
)
...(i)
Also,
l
µ
s
=
µ
s
µ
l
=
1.5
1.6
∴
1
f
′
=
(
l
µ
g
−
1
)
(
1
R
1
−
1
R
2
)
1
f
′
=
(
15
16
−
1
)
(
1
R
1
−
1
R
2
)
...(ii)
Dividing Eq. (i) by Eq. (ii), we get
f
f
′
=
(
15
16
−
1
)
(
1.5
−
1
)
=
−
1
16
×
5
f
=
−
160
c
m
© examsnet.com
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