When a fire engine approaches, the siren gets louder as it comes closer, taxing your eardrums as it whizzes past, and fades into the distance. What causes this difference in volume?
The answer was proposed by Austrian mathematician and physicist Christian Doppler in 1842 in a phenomenon since documented as the Doppler Effect, a concept that applies to both sound and light, in addition to other phenomena.
Simply put, the sound is generated in ‘waves.’ As the source of such waves moves closer, the waves themselves take less time to reach you. They hit you with increasing frequency, making the sound more intense. As the source moves away, the waves start to spread out, and the sound becomes weaker.
The Doppler effect also explains why stars in the sky appear to be of different colors. As a star approaches the earth, wavelengths compress and the star appears to be bluer in color. If the converse happens and a star is moving away from us, it appears redder.
Though the Doppler Effect is his most famous contribution to scientific literature, Christian Doppler authored over fifty works in mathematics, physics, and astronomy over the course of a twenty-year teaching career that spanned modern-day Austria, the Czech Republic, and Hungary.
Today’s Doodle by guest artist Max Löffler celebrates Christian Doppler’s 214th birthday. It shows Doppler in Salzburg, his native city, holding an airplane as it creates a Doppler effect.
Alles Gute zum Geburtstag, Herr Doppler!