Optics in Action 2 of 3
Is that light curling around that cylinder? Yes. Imagine this: A light ray enters the edge of the plastic. It travels through the plastic until it comes to the junction between the air and cylinder. Because of the small angle the light makes with the cup, instead of going through the junction and refracting, it reflects back in to the plastic. After reflecting, the ray hits another junction and reflects again. This process contains the light within the plastic as it continues on its initial downward direction, creating the helix you see. This is the same principle that fiber optics use to transmit light through non-linear paths.