We have all had the experience of trying to figure out whether police and fire sirens are headed toward us or away. A siren coming closer will seem have a higher pitch than one going away.
The change in pitch is called the Doppler effect , and the actual change in frequency is called the Doppler shift. It is named for Austrian mathematician and physicist Christian Doppler (1803–1853), who is credited for proposing it. It turns out that the Doppler effect applies to light waves as well as sound. Light from a source that ...
