Coherence Revision

• Coherent waves


  Have a constant phase difference and the same frequency
• Coherent light sources


  • Lasers
• The middle example on the right is not an example of a coherent light source
• Coherent


  Constant phase difference
• Endoscopes


  • Make use of total internal reflection
  • Light is transmitted through one bundle of fibres, and light is emitted at the other end
  • This light does not need to be coherent
  • Light that returns and is viewed does need to be coherent
  • Each fibre needs to be in the same relative position at one end of the endoscope as the other
• Coherent bundles


  • Bundle A
• Non-coherent bundles


  • Bundle B
• Coherent waves have the same wavelength or frequency
• Coherent waves have the same phase or constant phase difference
• Coherent


  • Used to transfer/transmit image (out of body)
  • Same fibre arrangements at both ends of bundle
• Non-coherent


  • Used to transfer light into body (to illuminate)
  • Random fibre arrangement along bundle
• sin θc = 1.49 / 1.52
• θc = 79°