doppler effect

Pronunciation

Noun B2 Upper-intermediate

The change in the pitch or wavelength of a wave that occurs because its source is moving relative to the observer.

"The doppler effect explains why an ambulance siren sounds higher as it approaches and lower as it drives away."

Usage notes

The same effect applies to light: astronomers use it to tell whether distant galaxies are moving toward or away from us.

Which word fits the gap? "The _____ explains why an ambulance siren sounds higher as it approaches and lower as it drives away."

Answer: doppler effect

In your own words, what does "doppler effect" mean?

The change in the pitch or wavelength of a wave that occurs because its source is moving relative to the observer.