Total Internal Reflection
Hundreds of thin plastic optical fibers fan out from a lamp, and each one glows only at its tip. The light goes in at one end and comes out at the other, even though the fibers bend and droop on the way. Inside each fiber the light hits the wall at a slant and reflects completely, as if from a perfect mirror, again and again, so none escapes from the sides.
Look along one fiber: is it lit along its length or only at its tip?
Additional Information
This is exactly how fiber-optic cables carry internet and phone signals across oceans — light bounces along the inside of a thin glass fiber, kilometer after kilometer, with amplifiers along the way.
It only happens past a critical angle; at shallower angles the light escapes through the surface instead of reflecting.
A diamond’s sparkle comes from the same effect — its cut angles are designed to trap and bounce light back out through the top.
In your textbook