Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
Austrian physicist Christian Doppler (1805-1853) derived the mathematical reliationship describing the change in the frequency of sound observed when source and receptor are moving relative to each other. We are familiar with the increasing pitch of a train whistle as the train approaches, and the decreasing pitch when it has passed and is moving away. The sound waves are crowded together and then strung apart. The same principle applies to electromagnetic waves, for example in astronomical observations of distant galaxies or other objects which show spectral shifts in their emission of light. Red shifts of galactic spectra indicate that the galaxy is moving away, a sign that the universe is expanding. The motion of weather systems can likewise be detected by constantly reflecting radio waves. The weather report references to Doppler radar are now commonplace.