# What is the Doppler effect?

The Doppler effect, also known as Doppler shift, is a phenomenon that occurs in any type of wave, both in electromagnetic waves, for example, light, and in mechanical waves, for example, sound, when there is relative movement between the source emitting the waves and the observer or receiver.

The phenomenon was first described in 1842 by the Austrian physicist Christian Doppler, and consists of an apparent change in frequency and wavelength caused by the relative motion between emitter and receiver. If the emitter approaches the receiver, the wavelength appears to decrease and the frequency increases. If the emitter and receiver move apart, the wavelength appears to increase and the frequency to decrease. All this without the emitted wavelength and frequency undergoing any change during the displacement of the emitting source.

## Doppler effect in sound

In everyday life and according to human perception ability, the Doppler effect can be easily experienced in some types of mechanical waves. For example, in sound. One of the most typical examples is the sirens of an ambulance.

Suppose we are standing on the sidewalk of a street and an ambulance passes by at 100 km/h, 8% of the speed of sound (1235 km/h).

As the ambulance approaches us, we hear the sound getting louder and louder due to the shortening of the sound wave length. But it’s an apparent shortening. Each wave emitted in front of the ambulance is closer to the previous wave due to the displacement of the ambulance itself in that direction.

If the ambulance moves at a speed equivalent to 8% of the speed of sound, the wavelength will be reduced by 8% in relation to the actual wavelength emitted by the siren. On the frequency it has the opposite effect, it would be increased by 8%, which is why we hear the siren with higher tones.

Once the ambulance passes us and pulls away, we start to hear the sound getting deeper and deeper due to the relative lengthening of the wavelength and decreasing of the frequency. Behind the ambulance, each wave emitted would be 8% further from the previous one.

If the ambulance could travel at exactly the speed of sound, there would be no sound waves in front of it. The ambulance itself will move alongside the sound wave front, and the wavelength will be offset by the distance traveled by the ambulance itself. We wouldn’t hear the ambulance until it was right in front of us.

If it could move at supersonic speeds, the sound wave front would even be behind the ambulance, as the waves would be slower than the ambulance and would be overtaken as soon as they were emitted.

In the following gallery you can see an animation of the Doppler effect on sound waves emitted by a stationary emitter and on displacements at subsonic, sonic and supersonic speeds.

### Gallery

Doppler effect, stationary emitter Doppler effect, emitter moving at subsonic speeds Doppler effect, emitter moving at the speed of sound Doppler effect, emitter moving at supersonic speeds

## In the light

To experience the Doppler shift in light and other electromagnetic waves, much greater relative displacement speeds between the emitter and the observed are required, since the speed of light is much greater than the speed of sound.

The changes in the wavelength of light are very small at the normal speeds that we humans experience. Even at speeds at which stars and galaxies move, high-sensitivity devices are needed to detect the Doppler shift in their lumen.

Precisely in astronomy, the Doppler effect is responsible for the redshift of light from distant stars and galaxies. These stars and galaxies move away from us at great speed and their light reaches us shifted to longer wavelengths, that is, to the area of ​​the electromagnetic spectrum corresponding to red light.

The redshift in light from distant stars and galaxies is one of the first observational evidence that the Universe is expanding today. And not only that, the observed redshift in the Universe also indicates that the farther a galaxy is, the faster it is moving away from us, which is consistent with the inflationary universe theory.

In the case of the displacement of electromagnetic waves in a vacuum, the Doppler effect is just a consequence of the relative displacement between the source and the observer. In the case of waves that necessarily need a medium to move, as is the case with mechanical waves that make up sound, the characteristics and movement of the medium can also affect the Doppler effect.

## formulas

Suppose an observer at rest and a source emitting waves with a given frequency f0. The wave propagation speed is independent of the source displacement and the observer speed is zero. Then, the frequency observed at a given time is described by the following formula:

Where:

f is the observed frequency and f0 the real frequency of the wave emitted by the source c is the propagation speed of the wave vs is the displacement speed of the emitter

If the observer is in motion and the source is at rest, the previous formula would be as follows:

Where Vr would be the displacement speed of the observer or receiver.

Both cases, as well as the moving source and receiver case, would respond to this general formula:

## Applications and implications of the Doppler effect

The Doppler effect has important applications and implications in different fields of science and technology. Some of the most notable, to name a few, are radar and sonar, the study of the Universe, communication between satellites or medical applications.

In radar and sonar, the Doppler effect allows you to measure the object’s scrolling speed. This is the basis, for example, of the traffic radars that control the speed of movement of vehicles. The radar continuously emits waves at a certain frequency; the change in the frequency of the waves reflected by the moving vehicles is what makes it possible to calculate their speed thanks to its relationship with the Doppler effect.