For an isosceles prism of angle A and refractive index μ , it is found that the angle of minimum deviation δm=A. Which of the following options is/are correct?
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a
For the angle of incidence i1=A, the ray inside the prism is parallel to the base of the prism
b
At minimum deviation, the incident angle i1 and the refracting angle r1 at the first refracting surface are related by r1=i1/2
c
For this prism, the emergent ray at the second surface will be tangential to the surface when the angle of incidence at the first surface isi1=sin−1sinA4cos2A2−1−cosA
d
For this prism the refractive index μ and the angle of prism A are related as A=12cos−1μ2
answer is A.
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Detailed Solution
δm=A & μ=sinA+δ¯m2sinA/2⇒μ=sinAsinA/2⇒μ=2sinA/2cosA/2sinA/2=2cosA/2⇒cosA/2=(μ/2)⇒(A/2)=cos−1(μ/2)⇒A=2cos−1(μ/2) (D) is not correct. δm=i1+i2−A⇒i1+i2=2A and i1=i2=ASo option A is correct.r1=r2 and μ=sini1sinr12cosA/2=sinAsinr1⇒sinr1=2sinA/2cosA/22cosA/2⇒r1=A/2=i1/2 option (B) is correct Emergent ray tangential to the surface meansi2=90∘⇒r2=C (critical angle) r1=(A−C)μ=sini1sinr1sini1=μsinr1=2cosA/2[sin(A−C)]=2cosA/2(sinAcosC−cosAsinC)=2cosA/2sinA1−sin2C−cosA1μ=2cosA/2sinA1−1μ2−cosA1μ=2cosA/2sinA1−14cos2A/2−cosA12cosA/2=2cosA/2sinA4cos2A/2−12cosA/2−cosA12cosA/2=sinA4cos2A/2−1−cosA⇒i1=sin−1sin−1A4cos2A/2−1−cosAOption C is correct