Doppler Effect

What is Doppler Effect?

The change in observed frequency of a wave caused by relative motion between its source and an observer — compressed and higher-pitched when approaching, stretched and lower-pitched when receding. It applies to any wave, which is why astronomers use its light-based version to measure whether distant galaxies are moving toward us or away.

The five clues

Engineerdle #18 described Doppler Effect with five clues, revealed one at a time from hardest to most direct. Each clue points at a different property of the concept.

1. 01A shift you've almost certainly heard, even if you never knew its name.
2. 02An ambulance siren sounds higher-pitched approaching than it does receding.
3. 03The effect applies just as well to light as it does to sound.
4. 04Astronomers use its light-based version to tell whether distant galaxies are approaching or receding.
5. 05The change in observed frequency of a wave caused by relative motion between the source and the observer.