Christian Johann Doppler He was an Austrian mathematician, born in 1803 in Salzburg, and died in Venice in 1853. He became famous for discovering the physical phenomenon called the Doppler effect. Educated at the Vienna Polytechnic Institute, he later became director of the Institute of Physics and professor of Experimental Physics at the University of Vienna. He wrote his first works in the field of mathematics but, in 1842, published a work entitled Concerning the colored light of double stars (about the colors of the light emitted by the double stars), in which he presents the fundamentals of the Doppler effect, both with sound and light.
Doppler observed that the length of a sound wave produced by a moving source changes. When the source is approaching the viewer, the wavelength decreases (ie the sound becomes sharper); when she moves away, he gets bigger (gets more serious).
A few years later, an experimental demonstration was made to confirm the validity of this formulation: they accommodated several buglers on an open wagon pulled by a locomotive. Several speeds were used, giving a different note each time. Meanwhile, on the station platform, several ear-eared musicians recorded the notes they could hear. The results confirmed the predictions calculated with the Doppler equation.
This phenomenon, known to this day as the Doppler effect, is not unique to sound, but also manifests itself in electromagnetic waves. Doppler even predicted that it would be valid for light, but this could only be properly explained later by the French Fizeau. In the case of light, the Doppler effect has proved particularly useful for astronomers in analyzing light emitted by distant bodies in space. (When the source moves away from the viewer, an object will appear redder than normal.) Applying the effect to these cases reinforced the theory of an expanding universe.