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Doppler Effect
Frequency Shift, Moving Sources, and Real-World Examples
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The Doppler effect is the change in observed frequency or wavelength of a wave when the source and observer move relative to each other. It helps explain why a siren sounds higher in pitch as an ambulance approaches and lower after it passes. This idea matters because it connects everyday experiences to wave physics and is also used in astronomy, weather science, and medical imaging. The same core principle applies to sound waves in air and light waves in space, although the details differ.
For sound, motion changes the spacing of wavefronts traveling through a medium, so an observer detects a different frequency than the source emits. If source and observer move closer together, the observed frequency increases, and if they move apart, it decreases. For light, the effect appears as blueshift when wavelengths get shorter and redshift when wavelengths get longer. Scientists use these shifts to measure speeds of stars, galaxies, blood flow, and moving storms.
Key Facts
- Doppler effect means observed frequency changes because of relative motion between source and observer.
- For sound, approaching motion gives higher observed frequency and receding motion gives lower observed frequency.
- Doppler formula for sound: f' = f(v + vo)/(v - vs)
- In that formula, v is wave speed, vo is observer speed toward the source, and vs is source speed toward the observer.
- Wavelength and frequency are related by v = fλ
- For light, shorter observed wavelength is blueshift and longer observed wavelength is redshift.
Vocabulary
- Frequency
- Frequency is the number of wave cycles that pass a point each second, measured in hertz.
- Wavelength
- Wavelength is the distance between matching points on consecutive waves, such as crest to crest.
- Wavefront
- A wavefront is a line or surface connecting points on a wave that are in the same phase.
- Redshift
- Redshift is an increase in observed wavelength of light caused by a source moving away.
- Blueshift
- Blueshift is a decrease in observed wavelength of light caused by a source moving closer.
Common Mistakes to Avoid
- Using the Doppler effect to claim the source frequency itself changes, which is wrong because the emitted frequency stays the same and only the observed frequency changes for a moving source or observer.
- Ignoring the sign convention in f' = f(v + vo)/(v - vs), which is wrong because choosing the wrong signs can predict a lower frequency for an approaching source.
- Assuming sound and light use exactly the same formula, which is wrong because sound usually depends on motion through a medium while light is treated with redshift and blueshift ideas from relativity.
- Thinking louder sound means higher frequency, which is wrong because loudness depends on amplitude while pitch depends on frequency.
Practice Questions
- 1 A stationary ambulance siren emits f = 800 Hz. A person in a car moves toward the ambulance at vo = 20 m/s. If the speed of sound is v = 340 m/s, what frequency does the person observe?
- 2 A train horn emits f = 500 Hz while the train moves toward a stationary observer at vs = 30 m/s. Take v = 340 m/s. What frequency does the observer hear?
- 3 A galaxy's light is observed to be redshifted. Explain what this tells you about the galaxy's motion relative to Earth and how the observed wavelength compares with the emitted wavelength.