Reflection can be a good thing ... is a circular polarizing filter, which can block or reduce light reflected at certain angles from the glass (or water or smooth nonmetal surfaces).

The normal is a construction line drawn at right angles to the surface of the glass block. Angle of incidence ... Light is bent more by glass than by water because glass is denser than water ...

Just like ray diagrams showing reflection ... bend when they are viewed through water, like the straw shown here. If the light hits the surface of a glass block at an angle, this change in ...