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Total Internal Reflection

When light is passing from one material to another it can be reflected back, trapping the light in the first material.

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Light Pipes

Light Pipes

Credit: Tobias Brixen
Source: http://www.flickr.com/photos/brixendk/4349817216/
License: CC BY-NC 3.0

Made of a flexible fiber, optical fibers are waveguides that allow light to be transmitted long distances with little to no loss of information.

Amazing But True

  • Optical fibers are cylindrical waveguides that are used to transmit light down the axis of the fiber. The optical fiber consists of two parts; an inner core and a surrounding layer with a lower index of refraction. This difference in the refractive indexes allows any pulses of light that are sent down the waveguide to stay within the inner core. The process of total internal reflection allows the light to be transmitted down a cylindrical waveguide . 
  • Total internal reflection is the process in which light that is traveling in a medium hits a boundary at such an angle that the light is completely reflected. Looking at Snell's law, it is easy to see under what conditions and at what angles this would happen

\begin{align*}n_1 \sin \theta_2 & = n_2 \sin \theta_2 \\ \theta_1 & = \sin^{-1} \left(\frac{n_2}{n_1}\right) \end{align*}

  • Therefore, total internal reflection will occur when light is incident on the boundary of a second medium with a lower index of refraction and the incident angle is greater than the critical angle.

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Using the information provided above, answer the following questions.

  1. What is the purpose of having an inner and outer core in the optical fiber?
  2. If light is traveling from a medium with an index of refraction equal to 1.33, into air, at what minimum angle would total internal reflection be seen?
  3. How does the impurities in the material an optical fiber is made out of affect how much information is lost?

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  1. [1]^ Credit: Tobias Brixen; Source: http://www.flickr.com/photos/brixendk/4349817216/; License: CC BY-NC 3.0

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