How to Use This Doppler Effect Applet
This simulation helps students connect relative motion to
wavefront spacing, detected frequency, and the timing of
wave detection. The red source emits sound waves, the green observer
receives them, and the applet lets you compare the space view
with the timing graph.
How to Use the Simulation
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Start with Baseline: source and observer stationary so you can
see the no-shift case first.
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Use the preset menu to compare a moving source, a
moving observer, and cases where both are moving.
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Use Play for the full motion, Step for slow reasoning,
and Reset to restart the case cleanly.
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Adjust
period, v source,
v observer, and c one at a time.
-
Turn on emit/detect to compare emission times with detection times.
Suggested Learning Sequence
- Run the baseline case and compare f with f'.
- Test a source moving toward and away from the observer.
- Test an observer moving toward and away from the source.
- Use emit/detect to explain why the detected rate changes.
- Try the sonic-boom extension only after the subsonic cases make sense.
What Students Should Notice
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In the baseline case, the wavefront spacing is uniform and the detected
frequency matches the emitted frequency.
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When the source moves toward the observer, the wavefronts in
front are compressed.
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When the source moves away, the wavefronts near the observer
are more spread out.
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A moving observer can also change the detected frequency even when the
source stays the same.
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At and above the sound speed, the model becomes an extension case for
discussing shock waves and sonic booms.
Suggested Inquiry
Predict first, then test:
Which changes the detected frequency more clearly in this model:
moving the source, moving the observer, or both moving together?
Quick Quiz for Students
These questions work well after students explore the presets and the
emit/detect view, or after watching the tutorial video.
Question 1
In the stationary baseline, how does the detected frequency compare
with the emitted frequency?
- It is the same.
- It is always higher.
- It is always lower.
Answer
A. It is the same.
Question 2
When the source moves toward the observer, what happens to the
wavefronts in front of the source?
- They spread farther apart.
- They get closer together.
- They disappear.
Answer
B. They get closer together.
Question 3
If the source stays the same but the observer moves toward the source,
what happens to the rate of detection?
- It decreases.
- It stays unchanged.
- It increases.
Answer
C. It increases.
Question 4
In the emit/detect panel, what do the green marks represent?
- Emitted waves
- Detected waves
- Source speed
Answer
B. Detected waves.
Question 5
If the period becomes longer, what happens to the emitted frequency?
- It increases.
- It decreases.
- It stays the same.
Answer
B. It decreases.
Question 6
Why is the supersonic case treated as an extension example?
- Because the source can outrun some of its own wavefronts.
- Because the observer disappears.
- Because sound speed no longer matters.
Answer
A. Because the source can outrun some of its own wavefronts.