1842 – Austria
‘Any source of sound or light moving away from an observer changes in frequency with reference to the observer’
The pitch of the whistle of a train is higher when the train is approaching an observer standing on a platform and lower when it is moving away from the observer.
Doppler explained the effect by pointing out that when the source of sound is moving toward the observer, sound waves reach the ear at shorter intervals, hence the higher pitch. When the source is moving away the waves reach the ear at longer intervals, hence the lower pitch. The Doppler effect also occurs when the source of sound is stationary and the observer is moving.
Doppler predicted that a similar effect would apply to light waves.
Different colours are the optical equivalent of notes of different pitch; blue light vibrates at roughly twice the pitch of red light.
In 1929 EDWIN HUBBLE suggested that the Doppler effect applied to light coming from distant stars gives a measure of the distance and speed of distant galaxies.