Q.

The refractive index of a prism with apex angle A is cot A2. The angle of minimum deviation is:

1 δm=180°-A  
2 δm=180°-3A  
3 δm=180°-4A  
4 δm=180°-2A  

Ans.

(4)

μ=sin(A+δm2)sinA2

cosA2sinA2=sin(A+δm2)sinA2

sin(π2-A2)=sin(A+δm2)

π2-A2=A2+δm2

δm=π-2A