Light from a bright point source with condenser and iris passes through a yellow filter and two crossed polaroids onto a distant screen. The quarter-wave plate is then positioned between the two crossed polaroids and rotated to any angle except 45 degrees with respect to the crossed polaroids. When the analyzer is rotated periodic minima and maxima of illumination appear on the screen, because the beam is elliptically polarized, and no position of the analyzer will produce darkness. When the polarizer and analyzer are crossed and the quarter-wave plate is set at 45 degrees, rotating the analyzer does not produce any changes in the intensity of the beam. This is the effect of circular polarization.
Note that the yellow filter is used because the thickness of the quarter wave plate is based on the wavelength of yellow light.