Why does a ray of light not bend when it goes from rearer medium to denser medium when it is normal to the plane?
When incident ray is normal to the plane(interface) ,refractive ray passes without bending.
To explain it there is need to know Huygens' principle.
According to it wavefront of the incident light fall simultaneously on the interface then propagate with different time delay without changing its wavefront in other medium.
Hence its speed changes without bending.