A Simple Experiment Perfectly Illustrates How Fiber Optic Cables Work

The best experiments are often the simplest.

by Sarah Emerson
Apr 18 2016, 4:55pm

Image: YouTube/Screenshot

Fiber optic cables connect the world by making communication possible. They're in our homes, workplaces, hospitals, and even at the bottom of the sea. But most of us don't often stop to think about how, exactly, they work.

Thankfully, a short but sweet video out of the University of Utah's Department of Physics and Astronomy has perfectly visualized the physical properties at work inside of a fiber optic cable.

According to the video's description, professors wanted to show their physics students how these types of cables work, but in a more exciting way than simply waving one about and pointing out its various parts.

To illustrate their lesson, professors first filled an empty bottle with water. A hole was poked in the side of the bottle and a laser pointer was positioned directly across from it, so that the beam exited precisely where the hole was made. When the bottle was squeezed, water poured out of the incision. And the laser beam, instead of remaining a straight line, bounced off the surface angle of the water spouting out of the bottle, creating a contained, liquid ray of light.

This is caused by something called total internal reflection, which allowed the laser beam to reflect off the interface between air and water. According to YouTube's engineerguy (whose deep dive into fiber optics I highly recommend watching), this occurs because of a "difference between the index of refraction of the guide material…and the outside [air]."

Whenever light hits a surface, it can be absorbed, reflected, or refracted by that material. But, as engineerguy explains, if a light ray hits a surface at an angle greater than the critical angle, it will be reflected and not refracted, which is exactly what happened in the University of Utah's experiment.

Fiber optics employs this same rule to transmit light signals over incredibly long distances. Instead of containing the light within a stream of water, however, optical fibers are made of glass (SiO2), glass plus polymers, or simply just polymers. They're thinner than the width of a human hair, with diameters ranging between 750 to 2000 microns.

One cable can contain up to several hundred fibers, which each carry information using light-based technology. When light travels down an optical fiber, it constantly reflects against the strand's inner mirror-lined cladding. Electrical information being sent from a source is coded into light pulses, which is then processed by a receiver that turns the pulses into information such as a live feed of your intestines, if we're talking about a fiber optics cable like an endoscope.

Fiber optics cables also work by transmitting sound for telephone landlines, or radio waves for cellphones.

There are a host of advantages to using fiber optics cables over traditional copper wires for our communications needs. Not only are they cheaper to manufacture, they're more effective, use less power, and are much safer. And if you ever want to show someone how they work, all you need to do is drink a liter of soda and get yourself a laser pointer.