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Combination of Mirror and Lenses
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The focal length of a convex lens of gla
Question
The focal length of a convex lens of glass
(
ΞΌ
=
1
.
5
)
is
2
c
m
. The focal length of the lens when immersed in a liquid of refractive index
1
.
2
5
will be :
A
5
c
m
B
2
.
4
c
m
C
1
c
m
D
4
c
m
Medium
Open in App
Solution
Verified by Toppr
Correct option is A)
Casr
1
lens in air
Let the focal length of lens in air be fair
Given that:
F
a
i
r
β
=
2
Β
c
m
,
Β
n
1
β
=
1
(
a
i
r
)
,
Β
n
2
β
=
1
.
5
(
g
Β
c
a
s
e
)
F
a
i
r
β
1
β
=
[
n
1
β
n
2
β
β
β
1
]
[
R
1
β
1
β
β
R
2
β
1
β
]
2
1
β
=
[
1
1
.
5
β
β
1
]
[
R
1
β
1
β
β
R
2
β
1
β
]
2
1
β
=
0
.
5
[
R
1
β
1
β
β
R
2
β
1
β
]
.
.
.
.
.
.
.
(
1
)
Case
2
lens in water
F
w
a
t
e
r
β
Β
β
Β
n
1
β
=
1
.
2
5
,
Β
n
2
β
=
1
.
5
F
w
a
t
e
r
β
1
β
=
[
n
1
β
n
2
β
β
β
1
]
[
R
1
β
1
β
β
R
2
β
1
β
]
F
w
a
t
e
r
β
1
β
=
[
1
.
2
5
1
.
5
β
β
1
]
[
R
1
β
1
β
β
R
2
β
1
β
]
F
w
a
t
e
r
β
1
β
=
0
.
2
[
R
1
β
1
β
β
R
2
β
1
β
]
.
.
.
.
.
.
.
.
.
(
2
)
Dividing
(
1
)
by
(
2
)
we get
2
F
w
a
t
e
r
β
β
=
0
.
2
0
.
5
β
β
Β
F
w
a
t
e
r
β
=
5
Β
c
m
β
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