system
Given the following post, respond as if you're commenting on Reddit:
human
Why does light not bend when it hits perpendicular to a new medium?
gpt
Here's a symmetry argument: Which way would the beam get re-directed?  There is no preferred direction, so it wouldn't be consistent for the light to be deflected one way or another.

More precisely: If the situation is rotationally symmetric along the axis  along which the incoming light travels, the behavior of the outgoing light must be unchanged under rotations about that axis.  Consequently, the beam of light going into the new medium has to go along the same direction as the incident light was traveling in the situation you describe.