angle of refraction,ϕ .
For θ < c, = ϕ - θ with sinϕ / sinθ = μ or ϕ = sin-1 (μsin θ).
∴ = sin-1 (μsin θ) - θ This is a nonlinear relation.
The maximum value of is 1 = π/2 - c for θ = c and ϕ = π/2.
For θ > c, = π - 2θ decreases linearly with θ.
max = π - 2c = 2 = 21.