Photography Lighting Lesson - How Light Reflects Off Glass

From Liz Masoner, former About.com Guide

Because light reflects at a predictable angle, the best way to combat glare/reflection is to change your angle in relation to the glass.

Determining Angles
As we discussed in step one, if light is not otherwise modified it will reflect at an equal but opposite angle to the angle it strikes a glass surface. This means that we can predict the angle of the glare/reflection and avoid it.

Flat Glass - Flat surface glass is the simplest to work with. The angle at which light strikes the surface will predict the way the light reflects. Simply stand to the side a few inches and shoot at an angle (or tilt your flash) to avoid glare. If you are trying to avoid the reflection of something else on the glass just step to the side a bit closer to the reflected object and it should disappear from view. For very large reflections it is sometimes necessary to shoot from higher or lower to completely avoid the reflection.
Concave Glass - Concave surface glass is glass that curves inward, as though you are looking into a bowl. If faced with this type of surface you'll actually want to be in the center of the curve and facing directly at the glass to avoid reflection. The other locations you can stand are on the outside edge of the curve shooting directly into that curve (not angled towards the center), or stand on the outside edge of the curve and shoot through the opposite curve.
Convex Glass - Convex surface glass is glass that curves outward, as though you are looking at a glass bead or bubble. For a convex surface glass you can stand and shoot at almost any angle except straight on from dead center. The convex surface basically magnifies the reflection angle so that angles are more pronounced instead of equal but opposite.

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