It is not so easy a thing to detect as you would imagine. We have seen that it produces no deviation or error in direction. Neither does it cause any change of colour or Doppler effect; that is, no shift of lines in spectrum. No steady wind can affect pitch, simply because it cannot blow waves to your ear more quickly than they are emitted. It hurries them along, but it lengthens them in the same proportion, and the result is that they arrive at the proper frequency. The precise effects of motion on pitch are summarised in the following table:—
Changes of Frequency due to Motion.
Source approaching shortens waves.
Receiver approaching alters relative velocity.
Medium flowing alters both wave-length and velocity in exactly compensatory manner.
What other phenomena may possibly result from motion? Here is a list:—
Phenomena resulting from Motion.
(1) Change or apparent change in direction; observed by telescope, and called aberration.
(2) Change or apparent change in frequency; observed by spectroscope, and called Doppler effect.