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Waves · Quick Reference
Doppler Effect Cheat Sheet
In one line: the Doppler effect explains why a passing ambulance siren seems to change pitch — the relative motion between a wave source and an observer shifts the frequency you actually perceive.
Key Ideas
1What is the Doppler Effect?. The apparent change in frequency (and wavelength) of a wave due to relative motion between the source and the observer.
2Approaching Source. When a wave source moves toward an observer, the observed frequency increases (wavelengths compress in front of the source).
3Receding Source. When a wave source moves away from an observer, the observed frequency decreases (wavelengths stretch out behind the source).
4Doppler Effect for Sound. Explains why an ambulance siren sounds higher-pitched as it approaches and lower-pitched as it moves away, even though the siren itself never changes its actual frequency.
5Doppler Effect for Light (Redshift/Blueshift). Light from objects moving away from us shifts toward longer (red) wavelengths (redshift); light from objects moving toward us shifts toward shorter (blue) wavelengths (blueshift) -- a key tool in astronomy.
Worked Examples
An ambulance siren sounds higher-pitched as it drives toward you and lower-pitched as it drives away. Explain why, using the Doppler effect.
The relative motion compresses sound waves when approaching (higher pitch) and stretches them when receding (lower pitch)
Astronomers observe that light from a distant galaxy is shifted toward longer (redder) wavelengths. What does this suggest about the galaxy's motion relative to Earth?
The galaxy is moving away from Earth
Does the actual frequency emitted by a moving sound source (like a siren) ever actually change?
No, the source's true emitted frequency stays the same; only the observed frequency appears to change due to relative motion
Practice Yourself
Does an approaching sound source cause higher or lower observed pitch?
Higher
Does a receding sound source cause higher or lower observed pitch?
Lower
What is 'redshift' in astronomy?
A shift of light toward longer wavelengths, indicating the source is moving away
What is 'blueshift'?
A shift of light toward shorter wavelengths, indicating the source is moving toward the observer