Why does a siren change pitch?
As an ambulance approaches, the sound waves ahead of it bunch up—shorter wavelength, higher pitch. Once it passes, the waves stretch out and the pitch drops. For a source approaching at speed v, the heard frequency is f′ = f·V/(V−v), where V is the speed of sound.
How does it apply to light and technology?
Light does the same thing. Light from receding stars shifts toward red (redshift); approaching stars shift toward blue. Redshift is key evidence for the expanding universe and the Big Bang. Police radar guns, Doppler ultrasound for blood flow, and weather radar all rely on it.
FAQ
- Does it happen if the listener moves?
- Yes—moving toward the source also raises the perceived frequency.
- Is it related to sonic booms?
- When a source outruns its own sound, wavefronts pile up into a shock wave—a sonic boom.
Sources reviewed: 2026-10-25
References
- Doppler, C. (1842). Über das farbige Licht der Doppelsterne. Abhandlungen der Königlichen Böhmischen Gesellschaft der Wissenschaften.
- Halliday, D., Resnick, R., & Walker, J. (2018). Fundamentals of Physics (11th ed.). Wiley.