When light passes through an optically active medium, such as quartz or a sugar solution, the plane of polarization does not remain fixed. Instead, it is gradually rotated as the light travels forward. This twisting of the polarization direction forms a helical motion inside the medium — a direct consequence of circular birefringence, where left- and right-circularly polarized light move at slightly different speeds.
When such a material is viewed between crossed polarizers in white light, the rotation angle differs for each wavelength. Some colours are transmitted, others suppressed, and together they form brilliant interference patterns. If the sample is cylindrical, the effect appears as spiralling bands of rainbow colours, like a barber pole winding upwards around an axis. In more complex structures, the result can be elegant helical ribbons of colour, shifting as the light or viewing angle changes.
The phenomenon combines geometry and optics: the polarization twists like a helix, and the colour bands wrap around in striped, spiralling patterns that are both scientifically revealing and visually mesmerizing.