How Fiber Optics Work: Total Internal Reflection

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Updated on Oct 17, 2013

Total internal reflection is what happens when light enters a new medium, or material, at an angle larger than a critical angle. A critical angle is when the incident angle, or the angle at which light hits the new medium, which is less than 90° where some light travels through and some like is reflected.

Total internal reflection explains how fiber optics work. Optical fibers are cables or fibers made of strands of transparent glass that can be thinner than a human hair! They’re used to transfer light from one end of the fiber to the other (which can be thousands of miles away). The light transmissions that occur can carry a huge amount of information and data at near-instantaneous speeds, and are responsible for carrying much of the information on the Internet.


Observe total internal reflection.


  • Laser pointer
  • Large glass container (a fish tank is best)
  • Water
  • Milk
  • Dry-erase marker
  • Powdered drink mix
  • Protractor


  1. Fill the glass container with water and add milk until the water is cloudy enough to see the laser pointer when you shine it through.
  2. Turn on the laser pointer and point it at an upward angle from the bottom of the glass container up towards the water surface. You can use the side of the glass container if you’d like, but if it has a glass bottom, try that.
  3. Play with the angle at which the laser strikes the surface of the water until the laser is totally reflected back into the water and no light leaves through the surface of the water when it meets the air.
  4. Have a friend use the dry-erase marker to draw a vertical line on the side of your fish tank starting from the bottom of the tank to the point at which the laser meets the surface of the water.
  5. Have your friend use the protractor to measure the incident angle formed by this line and the path of the incident light emitted from the laser.
  6. What happens when you increase this angle? When you decrease it?
  7. Play with pointing the laser through the container at different angles and from different places. Record any observations you make, such as when light doesn’t come out of the top of the water, or when you get many reflections.
  8. Empty the water out of your container.
  9. Repeat the experiment using a container filled with drink mix. Do you notice anything different about the minimum angle necessary to create total internal reflection? How can you explain your observations?


If the incident angle is below the critical angle for water, light will pass through the surface of the water and into the air (but it will still get refracted). If the angle is greater than the critical angle, the light will display total internal reflection. All of the light is reflected back into the container.


Two important requirements for total internal reflection must be met. The first is the light must be passing from a denser medium, like water or glass, into a less dense medium, like air. The speed of light travels much faster in air than it does in water, explaining why light gets refracted, or bent, when it encounters a new medium. The second requirement is that the incident angle must be greater than the critical angle for the medium. This will be slightly different for the milky water than for the water mixed with powdered drink mix.

When these requirements are met, the light will travel through the denser medium in such a way that it is continually reflected inside the medium and does not pass through to the less dense medium (air). Fiber optic technology uses this phenomenon to continually reflect pulses of light carried down a flexible tube to be received by a sensor at the other side, which parses these on-off pulses into useful information (like the webpage you’re reading right now)!

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