A parallel beam of light falls on a solid transparent sphere. For what value of refractive index μ, the thin beam can be focussed at circumference of sphere :
1.5
2
2.5
none of these
A
2
B
1.5
C
2.5
D
none of these
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Solution
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Since,incident light is paralle,u=−∞ and v=2R
μ2v−μ1u=μ2−μ1R
Here, μ2=μ and μ1=1
⟹μ2R−1−∞=μ−1R
⟹μ2R=μ−1R
⟹μ=2μ−2
⟹μ=2
Answer-(B).
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