Funny things happen when light rays hit transparent media like glass or plastic at shallow (small) angles. In physics/optics this is known as the angle of refraction vs the angle of reflection, along with the "critical angle" where all light gets reflected rather than passing through the glass.
Here's a nice diagram to illustrate the phenomenon:
angle of incidence: angles of refraction and reflection on glass :: industrial glass -- Britannica Online Encyclopedia
This is not a perfect analogy, but pretty good: if you shoot a bullet at water at 90 degrees, it easily penetrates the water. If you shoot the bullet at the water at a very shallow angle, like five degrees, it will almost always "skip" and be "reflected" by the water. It can't penetrate the surface. Same deal with photons and glass/plastic. This turns out to have all kinds of repercussions for windshields and headlights with shallow/extreme angles of incidence.
I'm an optometrist, so this is a subject of some interest to me.
Finest regards,
troy