The apparatus consists of a bended rod of acrylic glass, 500 mm in length and 10 mm in diameter, with two windings of approx. 50 mm dia. The light incident on one face of the rod is guided through the rod and its windings by repeated total reflection and is emitted at the other face of the rod as a divergent light beam.
Due to the unavoidable inhomogeneities of the acrylic glass and to the somewhat small winding diameter as compared with the diameter of the rod, the total reflection is slightly disturbed. A small fraction of the light guided through the interior of the rod is emitted tangentially in the direction of the light, visibly for the observer, along the bent outer surface of the bending rod.