How Does Fiber Optics Work?

Light travels down a fiber-optic cable by bouncing repeatedly off the walls. Each tiny photon (particle of light) bounces down the pipe like a bobsleigh going down an ice run. Now you might expect a beam of light, traveling in a clear glass pipe, simply to leak out of the edges. But if light hits glass at a really shallow angle (less than 42 degrees), it reflects back in again—as though the glass were really a mirror. This phenomenon is called total internal reflection. It's one of the things that keep light inside the pipe.

Light Launched into the Fiber Core

The other thing that keeps light in the pipe is the structure of the cable, which is made up of two separate parts.

Fiber Optic Cable GYTA

The main part of the cable—in the middle—is called the core and that's the bit the light travels through. Wrapped around the outside of the core is another layer of glass called the cladding. The cladding's job is to keep the light signals inside the core. It can do this because it is made of a different type of glass to the core. (More technically, the cladding has a higher refractive index than the core. Light travels slower in the cladding than in the core. Any light that tries to leak into the cladding tends to bend back inside the core.)

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