• What is Doppler Effect and what are its applications?

You have probably noticed how the sound of moving objects varies depending on whether they are becoming closer or moving away from us. Well, this is the Doppler defect.

And we are not talking about the intensity of the sound but its tone, which can vary until it is unrecognisable. But let’s just take a step at a time.

In today’s post we look at what the Doppler effect is, what it consists of and above all, we will look at specific examples that’ll make it so much easier to understand. Are you ready? Let’s get started!

Understanding the concept of the Doppler effect

Doppler effect is the frequency change of a wave produced when the emitter is in motion. The receptor is stationery and perceives wave changes according to the distance between itself and the emitter, which is in motion.

The first reference we have about the Doppler effect is a study carried out by the Austrian physicist Christian Andreas Doppler, in 1842. In it, Doppler recounts the changes in sound tone of a train engine: its pitch is higher (its frequency increases) when it’s near and it becomes lower when it moves away.

Years later, in 1848, the french physicist Armand Hippolyte Louis Fizeau, discovered something similar occurring to electromagnetic waves. This is the reason why the Doppler effect is also known as Doppler-Fizeau.

Applications and examples of the Doppler effect

Ambulance sirens

The sound of the siren of an ambulance passing by is a really simple example that shows the Doppler effect. Although it might only be moving at 50 km/h, the change from low to high pitch can already be noticed exactly at the time it drives past us. The same is true of fire engines and police cars.

Astronomy measurements

The Doppler effect is also present in the visible spectrum of electromagnetic radiation. If an object moves away, its wavelength increases. If an object moves away, its wavelength increases producing redshift. In contrast, by becoming closer, its wavelength becomes shorter, producing blueshift.

Traffic radars

One of the main applications of the Doppler effect is in traffic radars, which measure the speed of cars thanks to this effect.

Radars send an electromagnetic signal and receive the rebound, this difference allows the calculation of the speed of the vehicle. The difference in frequency is also used to take the photograph.

The Doppler effect in aviation

As you can imagine, since we are talking about this theme, the Doppler effect is also used in aviation.

Similarly to traffic radars, onboard radars send out waves towards the ground’s surface to measure the speed of aeroplanes in flight. How? Well, once the wave is sent and its rebound received, the radar compares the frequency change to determine the speed of movement with respect to the ground.

It is also used by meteorological radars to detect the intensity of precipitation. If you’d like to find out more about this, we recommend taking a look at the linked post.

The Doppler radar started being used in aviation in 1961, and marked a great advance in long distance navigation which eliminated the need to carry a navigator on board and reduced the number or radio aids required.

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