Critical angle & optical fibres

• Critical angle


  The angle at which total internal reflection occurs
• The sparkle of diamonds is due to light being very effectively trapped in them by total internal reflection
• The critical angle for diamonds is 24.4°
• Total internal reflection


  The complete reflection of light at the boundary between two media with different refractive indices
• Light introduced to optical fibre

  1. At an angle higher than the critical angle

  2. Undergoes total internal reflection

  3. Very efficient, especially in comparison to copper wires

  4. Rays undergo total internal reflection within the core of the fibre
• Optical fibre


  • Core (middle) has a higher refractive index
  • Cladding (outside) has a lower refractive index
• Refractive index


  A measure of how much the speed of light is reduced when passing through a material
• Symbol for refractive index

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• Applications of optical fibres

  • Endoscopes
  • Broadband cables
• How an endoscope works

  1. Light is introduced into the fibre at an angle higher than the critical angle

  2. Undergoes total internal reflection

  3. Allows the image to be transmitted to the viewer