Polarization of Scattered Light

Also known as the “Blue Sky, Red Sunset Demonstration” this simple apparatus at once illustrates both the wavelength dependence and polarization of scattered light. A bright light source is projected through a tank of water onto a white screen. The transmitted light (falling on the screen) is observed to be somewhat more reddish and not polarized, while the light which is scattered at 90 degrees appears more bluish and is strongly polarized. Ordinary tap water should have enough air bubbles and debris to make an effective scattering demonstration. If the water is too clean, scattering can be enhanced by addition of a bit of milk, dish detergent, colloidal silica, or other suitable small particles that will remain suspended for a reasonable amount of time. Students may be provided with individual pieces of polarizing film, or a single large piece of polarizer can be held by the instructor in front of the tank to show the polarization effect.