The green laser entering the two columns of fluid is linearly polarized and passed through a rotating 1/2-wave plate. Karo syrup (LEFT column), which is high fructose corn syrup, has circular birefringence. Periodic dark bands spiral up the Karo syrup column. A water column is shown on the RIGHT for comparison. In both columns, the green glow that you see is light being scattered towards you by impurities and tiny density fluctuations in the liquid (Rayleigh scattering).
Before clicking on the Play button, the spiral in the Karo syrup column on the left is still and the light band along the water column is dark.
WATER COLUMN: When the polarization of the laser light entering the water column is pointed towards you, the green glow is minimal. As you play the video, rotating the 1/2 wave plate rotates the direction of the laser polarization entering the column. As the polarizarion begins to point perpendicular to your gaze, the vertical band of scattered light in the water column gets brighter and then fades again as the polarization is further rotated and becomes aligned with the direction of your gaze once more. This cycle is repeated twice in this video.
KARO SYRUP COLUMN: Unlike water, Karo syrup causes the polarization of the laser light to rotate as it propagates through it. As a result, the laser polarization direction periodically switches from being perpendicular to parallel to the direction of your gaze and produces gradually alternating bright and dark bands as the light propagates upwards, giving rise to a spiraling shape. In this case, the effect of the rotating 1/2 wave plate, which can be seen when you click the play button, is to move this spiral along the column. (The 1/2 wave plate is being rotated back and forth.)
Circular birefringence is a property of the chiral sugar molecules present in the Karo syrup but not in water. Circular birefringence causes a phase shift between left- and right-circularly polarized light traveling through the column. This phase shift is due a difference between the refractive index of left-circular and that of right-circular polarized light. Furthermore, the sum of left-circular and right circular light adds up to linearly polarized light pointed in a direction which depends on that phase shift. Thus, conversely, the linear polarization of the laser light entering the two columns of fluid may be decomposed into a left and a right circular component. In Karo syrup the phase shift between these two components increases as the light propagates in the syrup, producing a bright band every integer multiple of pi, when the resulting linear polarization is pointed perpendicular to your gaze. In water, the phase shift between the left and right circular components remains equal to zero.
What do you think will happen if white light were used instead of the green laser in this exhibit?
This demonstration was inspired by Pecina and Smith, J. of Chemical Education, 1999. 76(9): p. 1230-1233
© 2012, Boustany Lab,
If you find the contents on this page useful, please acknowledge the Boustany Lab in BME at Rutgers University.