Light reflection

Take a rubber ball and toss it at an angle at a flat surface, like a sidewalk or a wall. You will notice it bounces off the surface at the same angle as it struck the surface. In other words, the angle of incidence equals the angle of reflection.

When light strikes a smooth surface, like shiny metal, a polished mirror or the water of an unruffled lake, it acts in the same manner. The reflected light waves bounce off the surface looking much the same (or behaving in much the same way) as they did before they struck it. You can prove it. Look at a bright object in a mirror, and now look at the object, and you will not see a discernible difference (other than the fact that it's reversed).

Different materials have differing degrees of reflection. If the surface is not mirror smooth, light striking it will bounce off its rough projections and reflect at different angles. Polished metal will act like a mirror’s surface whereas lightly-brushed metal will diffuse much of the reflected light, producing a dimmer reflection. A cut diamond with its many surfaces will directly reflect some light from a polished facet but will also allow light to enter it and reflect from other facets at different angles. Water itself reflects some light but also absorb some of the light, so that a reflection of the sun off the surface of a deep lake will not be quite as intense as the sun itself. This is where polarized light comes in.

REFLECTIONS AND POLARIZED LIGHT

Light from the sun is composed of light waves that vibrate in all directions, which means the sun’s light waves have a rippled or wavy form to them, and they undulate in all directions. Such light is unpolarized.

Reflection from many surfaces eliminates some of these vibrations. For example, sunlight striking water or glass penetrates the surface and also reflects from it. The part that penetrates and continues through it remains basically unpolarized. The reflected light, however, is polarized - that is, its waves reflect in a single plane, oriented at one angle rather than many angles.

A polarizing filter can block light oriented at one angle, which means that reflected light from glass or water (even water molecules in the atmosphere) can be virtually eliminated through the proper use of a polarizing filter or polarizer. This degree of control is important for photographers to know, since a polarizing filter can significantly alter the look of a scene, and can, in many cases, eliminate reflections that can be seen with the naked eye.

Visit our section on using polarizing filters to learn how to use this characteristic of reflected light to your advantage in making pictures.