Polarizing Filters Thru Scattering Of Light

A ray of unpolarized light that strikes a molecule of air or other very tiny particle and is scattered by it becomes plane polarized perpendicular to the direction of propagation of the incident ray.


Sky Effects

  • The light of the blue sky is partially polarized by the same scattering that produces the blue color. If a polarizing filter is used to absorb the polarized rays, the blue color becomes noticeably darker. Because polarization is strongest at a right angle to the direction of propagation of a ray, the effect is greatest perpendicular to the direction of the sun, and nil looking directly toward or away from it. At sunset, the sky light at the zenith is strongly polarized, as is the sky light at the north and south horizons. At midday, strongest polarization is near the horizon and toward east or west. Maximum polarization effects are seen on very clear days, with hazier skies giving progressively less effect, as multiple scattering and consequent depolarization of the light increases.

  • Thus, a polarizing filter can darken parts of the sky without causing the color effects of yellow or red filters; consequently, the sky can be emphasized in a color photograph, or in black-and-white when no color filtering effects are wanted. Pictorially, the greatest disadvantage is that if you are not looking at right angles to the sun line (that is, a line toward or away from the position of the sun in the sky) you get little or no effect. If you are using a wide-angle lens to obtain a panoramic view, you will find that the sky tones will grade off as the angle of view across the picture becomes less than 90 degrees to the sun line; therefore, depending on the relative angles of the sun line and the lens axis, the sky may appear much darker at one end of the picture than at the other end; or, the sky might be dark at the center and light at the ends, or vice versa. Nevertheless, the sky effects that are obtainable may be very desirable. With black-and-white film, a color filter could be used with a polarizer to obtain a greater effect than either could give alone.

  • The easiest way to determine the visual effect of the light absorption of a polarizing filter is to look at the subject through it while rotating the filter. The effects will be visible and very apparent. When the desired degree of effect is obtained, place the filter on the lens at the same angle of rotation. With single-lens reflex cameras, or other ground glass viewing cameras, you can place the filter on the lens and look through the camera viewfinder to observe the effects of rotation. Manufacturers of some twin-lens reflex cameras provide an interlocked double polarizer that has a filter for each lens, so that the rotation of one produces a comparable rotation of the other.

  • If you wear Polaroid sunglasses while doing outdoor photography with a polarizing filter, you will see nothing at all through the viewfinder when you rotate the camera filter to a 90-degree angle to the polarization plane of those glasses, because virtually all the light will be absorbed when the two polarizers are so crossed. Take off the glasses and all will be well.

Haze Effects

  • Photographable blue atmospheric haze is due to the scattering of both blue light and ultraviolet radiation, and some of this blue light is polarized by the scattering process. Therefore, the penetration of distance haze can be maximized by using an ultraviolet or blue-absorbing filter, supplemented by a polarizing filter. The usefulness of the polarizer is limited by direction similar to the sky effects. And, to a degree, the hazier the day the less use a polarizing filter will be, because of the depolarizing effects of multiple scattering.

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