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Doppler effect

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The Doppler effect

Named after Austrian physicist Christian Doppler who proposed it in 1842.

You hear the high pitch of an approaching ambulance’s siren – and then notice that its pitch drops as it passes you. That’s the Doppler effect.

Listen to the Doppler effect! A passing car beeps its horn

Doppler effect Acela express train

Doppler effect racetrack

The Big Bang Theory – The Doppler Effect

Doppler effect YouTube example 1

Doppler effect YouTube example 2

sound frequency increases during the approach,
is identical as it passes by,
and decreases during the recession.
(Adapted from Wikipedia.)

Watch the spacing of the sound waves, when the car is at rest, and when it is in motion. How does motion change the spacing of the waves?



Step 1: Staying at rest

In the center is a stationary sound source.
It produces sound waves.
The wavefronts propagate symmetrically away from the source,
at a constant speed



Step 2: Now the source is moving quickly.

Since the source is moving, the centre of each new wavefront
is slightly displaced to the right.

As a result, the wave-fronts begin to bunch up in front of,
and spread further apart behind, the source.

So an observer in front of the source will hear a higher frequency.


We can see this in water: Doppler effect of water flow around a swan



Doppler effect applet

Doppler applet (with sound)

Doppler effect and sonic booms

MCAS problems

By the end of our unit on waves we should be able to do MCAS Physics exam: sample wave problems


Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling within various media. Recognize that electromagnetic waves can travel through empty space (without a medium) as compared to mechanical waves that require a medium

SAT subject test in Physics: Waves and optics

• General wave properties, such as wave speed, frequency, wavelength, superposition, standing wave diffraction, and Doppler effect
• Reflection and refraction, such as Snell’s law and changes in wavelength and speed
• Ray optics, such as image formation using pinholes, mirrors, and lenses
• Physical optics, such as single-slit diffraction, double-slit interference, polarization, and color

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