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CBSE Class 12 Physics 2023 Outside Delhi Set 3 Paper

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Question : 10
Total: 13
The power of a thin lens is +5D. When it is immersed in a liquid, it behaves like a concave lens of focal length 100‌cm. Calculate the refractive index of the liquid. Given refractive index of glass = 1.5
Solution:     👈: Video Solution
Power of the lens =+5D
Focal length of the lens =f=‌
1
D
Or, ‌f=‌
1
5
‌cm
∴‌f=20‌cm
Using lens makers' formula,
‌
1
f‌air ‌
‌=(‌
µ‌glass ‌−µ‌air ‌
µ‌air ‌
)(‌
1
R1
−‌
1
R2
)
Or, ‌
1
20
‌=(‌
1.5−1
1
)(‌
1
R1
−‌
1
R2
)
Or, ‌
1
20
‌=0.5(‌
1
R1
−‌
1
R2
)
∴‌‌(‌
1
R1
−‌
1
R2
)=‌
1
10
When immersed in liquid,
‌‌
1
f‌liquid ‌
=(‌
µ‌glass ‌−µ‌liquid ‌
µ‌liquid ‌
) (‌
1
R1
−‌
1
R2
)
‌ Or, ‌‌−‌
1
100
=(‌
1.5−µ‌liquid ‌
µ‌liquid ‌
) (‌
1
R1
−‌
1
R2
)
‌ Or, ‌‌‌−‌
1
100
=(‌
1.5−µ‌liquid ‌
µ‌liquid ‌
) (‌
1
10
) [Putting (
1
R1
−
1
R2
)=
1
10
 ]
‌ Or, ‌‌‌
1
10
=(‌
1.5−µ‌liquid ‌
µ‌liquid ‌
)
‌ Or, ‌‌9µ‌liquid ‌=15
∴‌µ‌liquid ‌=‌
15
9
=‌
5
3
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