The condition for refraction from medium1 to medium 2, when medium 1 is optically more dense than medium 2 :
we have snell's law: Sin i / Sin r = μ₂ / μ₁ ---- (1)
1) Condition for total internal reflection is : when r = 90 deg.
i = i_C = Sin ⁻¹ (μ₂ / μ₁)
If the angle of incidence is more than i_C then the light rays are reflected back into the medium 1 and do not refract.
2) When the light is travelling from medium 1 to medium 2 , the refractive indices can be related in any manner.
if angle of incidence = i = 0 , then angle of refraction r = 0 from formula (1)
So, there is no refraction.