Total Internal Reflection & Fiber Optic Cables

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  • 0:05 What Is a Fiber Optic Cable?
  • 0:38 Refraction and Snell's Law
  • 2:08 Total Internal Reflection
  • 3:00 How Do Fiber Optic…
  • 3:33 Lesson Summary
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Lesson Transcript
Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Fiber optic cables can transmit light very quickly over large distances. How exactly do they work and what does this have to do with total internal reflection? In this lesson, learn the answers to these questions and more!

What Is a Fiber Optic Cable?

What amazing device can transmit information at the speed of light, create beautiful displays of colored light, and also illuminate inaccessible places like the inside of your body? All of these things and more can be accomplished by fiber optic cables. Fiber optic cables transmit beams of light without allowing any of the light to escape until it gets to the end of the cable. How does this process work? To understand it, you first need to know something about how light bends when it passes from one material to another, a process called refraction.

Refraction and Snell's Law

Whenever light passes from one material into another, it changes speeds and bends. You've probably noticed this before whenever you try to look at something underwater. Imagine that you look down into a clear lake and see a fish under the water. If you try to reach down and touch the fish, you probably will miss it, even if the fish doesn't move at all. That's because the fish isn't really where it looks like it is. Because the light reflecting off the fish's body bends as it travels from the water to the air, your eyes can't really tell exactly where the fish is.

Snell's Law allows you to calculate exactly how much a ray of light will bend when it moves from one material into another. To use Snell's law, you need to know what the index of refraction (n) is for each material. The index of refraction tells you how much the light's speed changes in that material and, therefore, also how much it will bend. Materials with a higher n will always bend light more than materials with a lower n.

Snell's Law tells you how much a light ray will bend if you know the initial angle that the light hits the surface, and the indices of refraction of the two materials.

If light moves from a material with a higher index of refraction to one with a lower index of refraction (n2 < n1), the angle of refraction will always be bigger than the angle of incidence, meaning that the light will bend towards the interface between the two materials.

Total Internal Reflection

What happens if the light bends so much that the angle becomes 90 degrees or even larger? In that case, no light will actually be able to travel out of the material it started in, and it will instead be reflected back. When this happens, it's called total internal reflection. You can rearrange Snell's Law to find the minimum angle at which total internal reflection will occur. This is called the critical angle.

What would the critical angle be if a light ray was traveling from glass (n1 = 1.5) into air (n2 = 1)?

critical angle calculations

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