#propagation  the movement of a wave through a medium
  supertype:  #physical_phenomenon  a natural phenomenon involving the physics of matter and energy
  subtype:  #Doppler_effect__Doppler_shift  change in the apparent frequency of a wave as observer and source move toward or away from each other
     subtype:  #red_shift__redshift  (astronomy) a shift in the spectra of distant galaxies toward longer wavelengths; generally interpreted as the Doppler effect resulting from the velocity at which they recede as the universe expands
  subtype:  #wave_front  all the points just reached by a wave as it propagates

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