Today, the world celebrates the 214th birthday of Christian Doppler, the Austrian mathematician and physicist who came up with the Doppler effect idea in 1842.
Doppler learned that sound and light waves have a higher frequency if the source is moving towards the observer and lower when moving away, and published his observation in his best-known treaties Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels (On the coloured light of the binary stars and some other stars of the heavens). And furthermore, over the course of his twenty-year teaching career, Doppler authored over fifth other works in mathematics, physics, and astronomy.
Doppler’s hypothesis was tested and confirmed by the Dutch scientists Christoph Ballot in 1845 (sound waves) and by the French physicist Hippolyte Fizeau in 1848 (electromagnetic waves).
Naturally, the first Doppler effect related essays and scientific papers were available in the German and French languages. And it was only in the 1871, that the term was introduced to British readers in the English translation of Adolphe Ganot’s Elementary treatise on physics, experimental and applied where the Doppler Effect was explained: “When a sounding body approaches the ear, the tone perceived is somewhat higher than the true one; but if the source of sound recedes..the tone perceived is lower. The truth of this, which is known as Doppler’s principle.”
Followed by the introduction of the Doppler shift – the change of the frequency resulting from the Doppler effect – that led to the knowledge of redshifts and blueshifts in the spectrum of light, where when a source is moving towards the observer, the wavelengths are shifted to shorter wavelengths – towards the blue, and when the source is moving away, the wavelengths are shifted to longer wavelengths – towards the red. And the Doppler shift served as the base for Hubble’s and Humason’s mathematical formula for determining the distance between two galaxies by measuring their redshifts, to eventually prove the expansion of the universe, and lead to the development of the Big Bang Theory.
Nowadays, the Doppler effect is used in many technologies that benefit people – like the Doppler radar to measure the velocity of detected objects and to equally well serve police and meteorology departments, the Doppler navigation to measure the speed of flying object, and of course, the Doppler Echocardiogram or Ultrasound where the utilisation of the Doppler Effect allows the determination of the speed and direction of the blood flood and the frequency of the sound waves – the creation of images of the heart.