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📘 Lesson 5 of 7 · Waves

🚨 Doppler Effect

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.

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01 Key Concepts

What is the Doppler Effect?

The apparent change in frequency (and wavelength) of a wave due to relative motion between the source and the observer.

Approaching Source

When a wave source moves toward an observer, the observed frequency increases (wavelengths compress in front of the source).

Receding Source

When a wave source moves away from an observer, the observed frequency decreases (wavelengths stretch out behind the source).

Doppler 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.

Doppler 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.

02 Solved Examples

Example 1 An ambulance siren sounds higher-pitched as it drives toward you and lower-pitched as it drives away. Explain why, using the Doppler effect.
  1. As the ambulance approaches, each successive sound wave is emitted slightly closer to you, compressing the wavelength and raising the perceived frequency (higher pitch).
  2. As it moves away, each wave is emitted slightly farther, stretching the wavelength and lowering the perceived frequency (lower pitch).
Answer: The relative motion compresses sound waves when approaching (higher pitch) and stretches them when receding (lower pitch)
Example 2 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?
  1. A shift toward longer wavelengths is called redshift.
  2. Redshift occurs when a light source is moving away from the observer.
Answer: The galaxy is moving away from Earth
Example 3 Does the actual frequency emitted by a moving sound source (like a siren) ever actually change?
  1. The Doppler effect describes a change in the OBSERVED (perceived) frequency, not the source's true emitted frequency.
  2. The siren itself continues emitting at its actual fixed frequency throughout.
Answer: No, the source's true emitted frequency stays the same; only the observed frequency appears to change due to relative motion

03 Practice Questions

1Does an approaching sound source cause higher or lower observed pitch?
Higher
2Does a receding sound source cause higher or lower observed pitch?
Lower
3What is 'redshift' in astronomy?
A shift of light toward longer wavelengths, indicating the source is moving away
4What is 'blueshift'?
A shift of light toward shorter wavelengths, indicating the source is moving toward the observer
5Does the source's true emitted frequency change during the Doppler effect?
No, only the observed frequency appears to change

📄 Doppler Effect — Downloadable Worksheet

10 questions with a full answer key. Grab the PDF to print, or try the interactive version in your browser.

🗒 Cheat Sheet ⬇ Download PDF 🖥 Interactive Worksheet