The Doppler effect

Abstract

Early history of the Doppler effect An important chapter in the history of astronomical spectroscopy opened on 25 May 1842. On this day Christian Doppler (1803–53) (Fig. 6.1), the professor of mathematics at the University of Prague (then part of Austria), delivered a lecture to the Royal Bohemian Scientific Society entitled ‘Concerning the coloured light of double stars and of some other heavenly bodies’ [1]. By analogy both with sound and waves in the sea, Doppler maintained that light waves undergo a change in frequency of oscillation, and hence of colour, either when the luminous source or the observer is in motion relative to the aether (whose existence was at that time supposed necessary for the transport of light waves). He gave formulae for the frequency change ∆ ν when either the source or observer were in motion, and these amounted to a statement of the now familiar equation ∆ ν/ν 0 = V/c . Here V is the relative speed in the line of sight, c is the speed of light, and ν 0 is the light wave's frequency for sources at rest. Doppler then made two incorrect assumptions: first, that the radiation from stars was largely confined to the visual region of the spectrum, and secondly that the space motions of the stars were frequently a significant fraction of the speed of light. As a consequence, stars normally appearing white are seen instead as strongly coloured, either violet or red, depending on their approach towards or recession from Earth.