The translucent sheets and optical fibers are being lit from below.  Light enters one end with a relative small angle incidence.  Once inside the sheet/fiber the light experiences *[total internal reflections][1]* multiple times because it hits the sides of the sheet/fiber with a large angle incidence.  It's not until the light gets to the far end of the sheet/fiber that it hits an internal surface at a small enough angle of incidence that it is not totally reflected internally and manages to emerge.


  [1]: https://en.wikipedia.org/wiki/Total_internal_reflection