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When i = r = 0 ,  the incident light rays are perpendicular to the surface of the interface between the two media. Light rays enter from medium1 to medium2.
Mu2 sin r = Mu1 sin i           => r = 0  if  and when i = 0.

When i = 90 deg,    the light ray is moving along the surface of the boundary between the media. It does not enter 2nd medium. There is no question of bending.

if i=90, then r cannot be 0.