Friday, May 28, 2010

polarized light

A key difference between metals and dielectrics like glass and clear fluids is a difference in the way the molecules hold onto their electrical charge carriers called electrons that make up most of the molecule's volume. In metals, some of the electrons from each atom (or molecule) can move freely about in the bulk material. This is also the reason that metals make such good electrical conductors. Light waves differ from radio waves only in degree, not in type. Incoming light waves cause these electrons to vibrate strongly in such a way that they generate an electromagnetic field that opposes the entering light. Most of this energy is then used to regenerate a field in a different direction that appears as reflected light. Almost all of the light is reflected; only a small amount is absorbed and a negligible amount is transmitted unless the metal is very thin, e.g., gold leaf or a partially silvered mirror.

On the other hand, the molecules in a dielectric material hold onto its electrons tightly, which greatly restricts the electrons' ability to vibrate or otherwise interact with the incoming field. Most of the incoming light therefore goes into the bulk material, with only a small portion being reflected at the surface. That proportion being reflected depends not only on the incoming light's frequency and direction but also on the direction (polarization) the incoming light is vibrating. At a certain angle called Brewster's angle that depends on the electrical properties of both materials - both the stuff that the light is entering and that which it is leaving - all of the light that vibrates perpendicular to the surface of the material enters the bulk material and none of it is reflected. The light vibrating parallel to the surface has no such restriction, so all the light reflected from such an incident beam is polarized perpendicular to the reflecting surface. Because this reflected light is polarized, it can be blocked by a polarizing filter on the camera lens oriented perpendicular to the plane of polarization of the reflected light.

The angle at which the polarization is closely related to the relative speeds that light propagates through the two substances. The index of refraction of a substance is the inverse of the speed that light propagates through that substance. The index of refraction is almost but not quite independent on the frequency of light. Water has an index of refraction of 1.33 or about 4/3, so light propagates through water at about 3/4 the speed it would propagate through air (or vacuum). Brewster's angle for water is 53 degrees from normal (perpendicular) incidence. Window glass typically has a refractive index of about 1.5 and has a Brewster's angle of a bit more than 56 degrees. Even exotic glass selected for its high index of refraction seldom goes much above 1.9, with a Brewster's angle of about 62 degrees. With the possible exception of diamonds (n = 2.42, Brewster's angle of 67-1/2 degrees) you will seldom encounter a material that you would want to suppress reflection on that is outside this range.

For ordinary photographic purposes, you can get a decent level of attenuation of the reflected light if you get reasonably close to Brewster's angle, but this polarizing phenomenon is of little help to you if the offending reflections are grazing off the surface at nearly 90 degrees to the normal - typical of sunsets over water - or from surfaces nearly perpendicular to your camera's optical axis, such as straight-on at a polished marble wall. However, if all of your light comes from your studio equipment, you can mount polarizers on all of your studio lights as well as the camera's lens. That way, you don't have to depend on the surface polarization effect to kill offending glare.

The common angle of 45 degrees is close enough to Brewster's angle for optical components to make the reflective properties of these components annoyingly dependent on the incoming light's amount and direction of polarization. A circular polarizer is therefore necessary for cameras with such components. A circular polarizer has a regular linear polarizer on the side facing the incoming light but in addition has an extra element called a quarter wave plate mounted to the back. The quarter wave plate changes the polarized light coming through in such a way that, although the light is still polarized, a linear polarizing element will react to this light in much the same way as it would react to unpolarized light. This removes the dependency of subsequent elements on the way the incoming light is polarized.

You can also use the way a circular polarizer is constructed to tell it from an ordinary linear polarizer. The linear polarizer will extinguish glare regardless of which surface faces the subject, but the circular polarizer will extinguish glare only if it is mounted in the conventional way.


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