Relative refractive index is μ=μrϵr=4×1=2
Now, for complete reflection, θ>θc, where, θc is critical angle.
Now, critical angle is sinθc=μ1=21
⇒θc=sin−1(21)⇒θc=30∘
So, the angle of incidence for which the total intensity of incident wave will be reflected back into the same medium is θ>30∘ i.e., 60∘.