For an equilateral prism (A=60°), TIR occurs if the angle of incidence at the second face is greater than the critical angle C. For minimum μ, this happens at grazing emergence, where r₂ = C. Since A = r₁ + r₂, and for minimum deviation i=e, we get r₁=r₂=A/2 = 30°. For TIR, the angle of incidence r₂ must be > C. Sin(r₂) > Sin(C) => Sin(30) > 1/μ => 1/2 > 1/μ => μ > 2.