To demonstrate total internal reflection.
Plexiglass spiral with attached lamp.
Due to total internal reflection the light from the lamp remains mostly confined within the spiral plexiglass rod,and only exits at the end, where the angle between the surface and the incoming light exceeds the critical angle. Actually, some light escapes at scratches along the spiral, as can be seen in the photograph.
P. J. Ouseph, "Colgate-Plus" - An Imperfect Light Pipe, TPT 31, 405 (1993).