What do the following pictures have in common with you? Click on each question mark to reveal the image.

Each of these demonstrates the Doppler effect.

Other resources cover different types of wave behaviors including reflection, rarefaction, resonance, diffraction, and interference. Here, you will look at another wave behavior that incorporates all of the previous wave behaviors and has its own unique properties.

### The Doppler Effect

Our ears interpret the frequency of a sound wave as pitch, and our eyes interpret the frequency of a light wave as color. If the wave source is moving, it can cause a difference between the actual frequency of the wave being emitted and the frequency that is received by our eyes or our ears. This image shows a wave source moving to the left. Notice that the waves on the right are bunched closer together while the waves on the left are stretched out.

Source: Doppler Effect diagrammatic, Antilived, Wikimedia Commons

Watch the following video for a step-by-step description of how this works, and then answer the questions that follow.

Source: The Doppler Effect, paulie1982, YouTube

1. What happens to the wave in front of the moving source?

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The wave in front of the moving source gets compacted, and the waves become closer to one another.

2. What happens to the wave behind the moving source?

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The wave behind the moving source spreads out, and the waves become further away from one another.

3. How does this change the frequency and pitch in front of the moving source?

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The frequency and pitch increases in front of the moving source.

4. How does this change the frequency and pitch behind the moving source?

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The frequency and pitch decreases behind the moving source.

### Challenge question 1

What happens to the wavelength, frequency, and pitch if the object was not moving (stationary)?

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The wavelength, frequency, and pitch would be the same in all areas surrounding the wave since the object is not moving toward or away from you.

### Challenge question 2

What happens to the wavelength, frequency, and pitch if you are moving at the same rate as the object?

Interactive popup. Assistance may be required.

The wavelength, frequency, and pitch would be the same in all areas surrounding the wave since the object is not moving toward or away from you.

Now that you've had a brief introduction to the wave behavior seen in the Doppler effect, let's investigate how it works in relation to sound and light and in other applications.

Sources for images used in this section, as they appear, from top to bottom:

• Motorcycle police in Gorgie Road, Edinburgh, Kim Traynor, Wikimedia Commons
• Square-townsend-fledermaus, Velho, Wikimedia Commons
• DaveScottKUSIByPhilKonstantin, Philkon Phil Konstantin, Wikimedia Commons
• Army.mil-37690-2009-07-22-080724, Ashley Bateman, Wikimedia Commons