See ,this really a good question,let me explain
As per the question the angle of incidence is 0 degree,
this means that this is case of normal incidence.
AND in the case of normal incidence the snells law has failed,i.e this case does not follows the snells law,and hence no refraction occurs in this case .
Hence ,the ray goes undeviated.
NOW ,here you can easily see the the incident ray and the refracted ray are antiparallel,hence forth they make 180 degree.