Doppler Effect

Most people know about the Doppler Effect, some without knowing it, some had to go through a Doppler ultrasound, some of us even know it causes the slide of the sound in sirens passing by...

Definition: The Doppler effect (or Doppler shift), is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source.

So what does that mean?

Sound and light are waves:

To perceive one of these, we need an emitter, and an observer, for the siren example, the ambulance siren is the emitter, and the microphone recording the sound, is the observer.

But as the emitter moves toward the observer, the wavelength are shortened, once the ambulance has passed the observer, it gets further away from the observer, and the wavelength is getting longer.

When the wavelength is shorter, a sound will have a higher pitch, once the wavelength gets longer, the sound will have a lower pitch, thus creating the change you can hear when the ambulance passes by.

A representation of the wave of a moving emitter:

There are a lot of usage for this effect, but 2 are more known to the public:

• Car Speed Radar
• Doppler ultrasound

So when the police measure car speed to give you a ticket, they are using the exact same principle, the radar will beam a wave at a known frequency, and receive the beam reflected by the vehicle, having its wavelength impacted by the vehicle speed, Based on the difference, the device will determine the speed.

In the case of Doppler ultrasound used in medical imaging, it is actually used to measure the flow of blood in the body.

Finally, I would add that the expansion rate of the universe, is measured using redshift which are an example of the Doppler effect.

For more examples of its usage, and more detailed explanations, don’t forget to check Wikipedia page, which is quite complete!