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Byju's Answer
Standard XII
Physics
Lens Maker's Formula
The radii of ...
Question
The radii of curvature of lens are
+
20
c
m
and
+
30
c
m
. The material of the lens has refracting index
1.6
. find the focal length of the lens
(a) if it is placed in air, and (b) if it is placed in water
(
μ
=
1.33
)
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Solution
R
1
=
+
20
c
m
;
R
2
=
+
30
c
m
;
μ
=
1.6
a) If placed in air:
1
f
=
(
μ
g
−
1
)
(
1
R
1
−
1
R
2
)
=
(
1.6
1
−
1
)
(
1
20
−
1
30
)
⇒
f
=
60
/
6
=
100
c
m
b) If placed in water:
1
f
=
(
μ
w
−
1
)
(
1
R
1
−
1
R
2
)
=
(
1.6
1.33
−
1
)
(
1
20
−
1
30
)
⇒
f
=
300
c
m
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Q.
The radii of curvature of a lens are + 20 cm and + 30 cm. The material of the lens has a refracting index 1.6. Find the focal length of the lens (a) if it is placed in air, and (b) if it is placed in water (μ = 1.33).