Lessons â€ē 3 Waves and Optics â€ē 3.4 Geometric OpticsPrevious lessonNext lesson

3.4.3 Total Internal Reflection and Optic Fibres

By the end of this lesson you can:

  • Explain total internal reflection and the conditions needed for it
  • Calculate the critical angle from the refractive indices
  • Explain how optic fibres use total internal reflection to carry light signals
Find your course: NSW Year 11: Wave behavioursNSW Year 12QLD Unit 2: WavesVIC Unit 1: Light and heatAP Physics 2: Unit 13IB: C.3 Wave phenomena
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Learn the idea: a short explanation, then a check.
Image description
In certain cases when light tries to enter a less dense medium, it does not refract, in fact it reflects straight back in
Its the reason why fish tanks can appear like mirrors, and the water surface can appear mirro like from underwater

​Total internal reflection is a unique property of refraction as the wave enters a substance of lower refractive index, it reflects back in, obeying the Law of Reflection

​Watch the video below to understand this phenomenon, including the underlying mathematical principles.
📝Practise
Worked examples first, then problems to try, then worksheets.
🕹ī¸Explore
Test your predictions in a simulation.
Interactive
  1. Now go back to the interactive above, and adjust the refractive indices so the second one is less then the first. (use the third TAB)
  2. Manipulate the beam to get total internal reflection.
  3. Calculate the critical angle and then use the built in protractor to confirm your calculation.
Application
A common application of total internal reflection is optic fibres.
In the demonstration here, I simulate an optic fibre using a stream of water