Light and optical systems
Light travels in straight lines until a surface reflects it or a change of material bends it, and both follow fixed rules.
A worked example
A ray striking a flat mirror 35 degrees from the normal leaves 35 degrees on the other side of the normal, so the two rays sit 70 degrees apart.
The mistake people actually make
Angles in optics are measured from the normal, the line drawn perpendicular to the surface, not from the surface itself. Light bends because its speed changes between materials, so it does not bend while crossing one uniform material.