ELI5: How do fiber optic cables keep light trapped inside?

Fiber optic cables keep light trapped inside by bouncing it off the inner walls like a never-ending mirror.

Imagine you are in a long, dark hallway and you roll a bouncy ball forward at a slight angle. Instead of going through the wall, the ball hits the surface, bounces off, hits the other side, and keeps ricocheting all the way to the end of the hall. That is exactly what the light does inside a fiber optic cable.

Here is how it works in everyday life:

• The Glass Core: The center of the cable is made of super-pure, flexible glass as thin as a strand of hair. Light shines into one end and travels down this glass highway.
• The Trick with Angles: When you look straight down into a swimming pool, you can see right through the water to the bottom. But if you crouch low and look across the water at a flat, shallow angle, the surface suddenly looks like a shiny mirror. Fiber cables use this same trick. The light is shot into the glass at a very shallow angle so it cannot pass through the sides.
• The Trapping Coat: Wrapping around the glass core is a special second layer called cladding. This layer acts like the boundary of the water, forcing the light to bounce back instead of leaking out.