Blue Sky and Red Sunset


The blue sky and red sunset resulting from selective scattering of blue light from the solar beam is modeled using a parallel beam of light from a slide projector. The beam passes through a tank of clear water and falls on an opal glass screen. A scattering agent is slowly stirred into the water and the beam becomes visible in scattered blue light. At the same time the transmitted light is noticeably reddened. Coffee Mate makes an excellent scattering material; very little is required. The beam scattered horizontally can be shown readily to be polarized. A mirror or white card at 45 degrees above the tank makes the vertically scattered beam visible to the class with polarization at right angles to that of the horizontal beam.

Special Notes: It is instructive (if students have appropriate preparation) to rotate the mirror above the tank in an attempt to reflect the vertically scattered beam in the forward direction. Since the mirror is itself a polarizing device, one finds himself involved in a classical double scattering experiment.