Polarized Light

An icy road heading into the sun becomes a sea of fire. Farther south, the sun lays a blinding glare on water, hiding the fish within. Polarized sunglasses will cut the glare without noticeably affecting the overall brightness, allowing visibility for driving and vision down into the water. A bright reflection from a metal surface, however, is unaffected. Why?

Light has aspects of both waves and particles, but for this problem, it is the waves that are important. The waves are made up of electrical and magnetic fields, vibrating at right angles to each other and to the direction the light is traveling.

In most light, natural or artificial, the electrical fields occur in all the directions perpendicular to the direction of the light. If the sun is on the western horizon, for example, the light it sheds toward the east will have electrical fields vibrating up and down, north and south, and every direction in between. This light is called unpolarized light.

Now suppose the sun is somewhat above the western horizon, with a smooth water surface at the ground. Some of the light will penetrate into the water and some will be reflected. But if we look in more detail, it turns out that the electrical fields that are vibrating in the surface of the water (north-south) have trouble penetrating the water and are mostly reflected, while those that are partly perpendicular to the water penetrate easily and produce only a little reflection. As a result, both the reflected light and the light entering the water become polarized, which simply means that one direction of vibration of the electrical field dominates. Most of the light reflected from a horizontal surface will have its electrical field horizontal, and we say the light is horizontally polarized. Ice, glass, or any other polished surface that does not conduct electricity behaves the same way. (Metals, which do conduct electricity, do not polarize light on reflection. This used to be obvious with polarized sunglasses when cars had chrome bumpers.)

Scattered light, like the Rayleigh scattering in the sky we've talked about in earlier columns, is polarized very much as if it were reflected -- if the sun is on the western horizon, the light of the sky overhead will be polarized in the north-south direction. This is true, however, only for light scattered just once. In a cloud, for instance, the light reaching our eyes has normally been bounced around by several cloud droplets, and the polarization has been lost.

There are a number of natural materials in which the arrangement of atoms produces preferred directions for electrical fields. In some, such as calcite, the speed of light with one polarization will differ from the other. In others, the absorption is higher for one polarization than the other. Artificial materials of this second type can be made by several different methods, and the good ones will block almost all light polarized in one direction, while only slightly reducing the intensity of light polarized at right angles to the first direction. Natural light coming through a sheet of such a material will be strongly polarized.

Suppose you look through a sheet of polarizing material at the glare on an ice or water surface. If the polarizing axis of the material is horizontal, the horizontally polarized light of the reflection will pass through freely. But if the material is rotated ninety degrees, so that its polarizing axis is vertical, the horizontally polarized part of the reflected light will be stopped. Vertically polarized sunglasses are thus very useful in blocking glare from horizontal surfaces such as icy roads or water. They will have no effect at all, however, on reflections from vertical windows.

This sensitivity to orientation of the polarizing material can create problems. The world looked unreal and glassy through a pair of polarized sunglasses I once owned, strangely flat but at the same time glittery. I finally traced the problem to the orientation of the lenses --the directions of the polarizing axes differed in the two eyes. I would see bright reflections off leaves in one eye, but not the other. When the sunglasses were held up against the blue sky about ninety degrees away from the sun and rotated, the two lenses did not darken and lighten together, as they should have. This kind of difference in the images seen by the two eyes can produce headaches or loss of depth perception. Check the lens orientation in polarized sunglasses!