Hmmm.. The wave appears to be changing direction. It is refracting.
One way to think about why is to imagine the wavefronts as rows of
marching soldiers.
Marching from air to glass is like marching from a grass field into a
very muddy field. When they do this they will SLOW DOWN.
The soldiers at one end of the row will slow down before the others so the rows will swing round and change direction. When they leave the muddy field these soldiers will speed up first so swing round again.
