A tube of sugar water (.75g sugar / 1ml of water, 10ml Pinesol - make 2.75L to fill tube, Karo Syrup can also be used) is illuminated by a white light source with a linear polarizer on each side of the tube. Rotating one of the polarizers will cause different colors to be projected out the oposite side of the tube and the "rainbow" of colors travel up and down the tube. Color filters between the tube and white light source may also be used for additional interesting effects. 

Barber shop pole polarization effect

The white light source may be replaced with a diode laser and the beam appears to dim and nearly disappear, then reappear. Two linear polarizers on an optics bracket may be used to dim the laser so the effect is easier to see. 

Sugar water tube polarization with laser