What is the Doppler effect?

The Doppler effect is one of the most common physical principles and from which scientific and technological innovation has taken the most advantage throughout history, being a physical feature widely used in multiple areas, such as navigation, medicine, astronomy. or meteorology.

This effect, named after its discoverer Christian Andreas Doppler, a 19th century Austrian physicist, states that the perception of the frequency of a wave is different when the wave is approaching than when the wave is moving away.

First step: What is the frequency of a wave?

Any wave, whether light, sound, radio or sea waves, has a certain frequency, the frequency indicates how many times per second that wave repeats.

The unit of measurement for the frequency of a wave is Hertz (Hz), so a wave with a frequency of, for example, 20Hz means that it repeats 20 times per second.

The frequency of a wave is its most important characteristic, along with its wavelength and amplitude, but it is what determines its nature:

• If we talk about light, the frequency of light waves determines whether that light is visible light, infrared light (used in remote controls or thermal imaging cameras), or ultraviolet light (used to sterilize surfaces or forensic analysis).
• If we talk about electromagnetic waves, the frequency of the waves determines whether the waves are radio waves, microwave waves or X-ray waves.
• If we talk about sound, the frequency of a wave is what determines whether the sound is low (low frequencies) or high (high frequencies).

Now that the concept of wave frequency has been explained, we move on to the main topic, but not before clarifying a key concept: The speed of propagation of any wave is equal to the wavelength multiplied by its frequency.

speed = length x frequency

Basics of the Doppler effect

To explain the Doppler effect, sound waves are used, since it is the most common everyday case that we all know.

The Doppler effect is based on the principle that if the emitter of a wave is moving towards an observer, the speed of propagation of the wave will be the speed of the wave plus the speed of the displacement, since the movement in favor favors The wave reaches the observer sooner, while when the emitter moves away from the observer, the speed of wave propagation will be the speed of the wave minus the speed of the emitter, since countermovement delays the wave's arrival at the observer.

Applied to sound, whose speed under normal conditions is 344 meters per second.

• If a car approaches a person at 108km/h (30 meters/s), the sound emitted by the car approaches the person at 344 m/s of the sound + 30 m/s of the speed of the car = 374 m/s.
• When the car moves away from a person at 108 km/h (30 meters/s), the sound emitted by the car moves away from the person at 344 m/s of the sound - the 30 m/s speed of the car = 314 m/s.
• This causes that when the car approaches, the frequency of the wave is perceived as (100 x (374-344))/344 = 9% higher (sharper) while when it moves away the frequency is perceived as (100 x (314-344))/344 = -9%, that is, 9% lower, that is, more serious.

Therefore, the sound of a car approaching 108km/h becomes almost 18% more serious when it passes in front of us.

Now you know why ambulance sirens sound different when the ambulance is approaching than when the ambulance is moving away.

Applications of the Doppler effect

The case of sound is the most representative since it is the easiest for humans to perceive, since sound has a relatively low speed.

When we move to electromagnetic waves, such as light, radio, x-rays, microwaves, television, fiber optics, radiation, radars..., which move at the speed of light, this effect is imperceptible to humans, since If we apply the example of the moving car to its lights, the difference in the light when it approaches and when it moves away is only 0.00002%, so a human observer will never notice the difference, but an electronic sensor will.

And that is why sensors are used to take advantage of these characteristics of electromagnetic waves in multiple disciplines.

Medicine: Doppler ultrasounds are used to observe blood flow within the body. This is vital for diagnosing conditions such as blockages in the arteries, blood flow through the heart and blood vessels, and for assessing fetal health.

Meteorology: Used in Doppler radars to measure wind speed and direction. This helps predict the weather, especially for the detection of severe conditions such as tornadoes, by observing how raindrops and ice crystals move in the atmosphere.

Radar and Sonar: For both civil and military applications, the Doppler effect is applied to determine the speed of objects, such as vehicles, aircraft, submarines and other moving objects, improving the accuracy of navigation and threat detection.

Acoustic Engineering: Used to analyze how sound propagates or is perceived in motion, which has applications in the design of concert halls.

Astrophysics: It is used to determine the movement and speed of approach or distance of stars and galaxies with respect to the Earth. By analyzing the change in the wavelength of light (redshift or blueshift), astronomers can infer whether a celestial object is moving away or closer and at what speed.

It is also used to study the expansion of the universe, including the redshift of distant galaxies, which indicates that the universe is expanding.

Automotive industry: In the development of radar systems for driver assistance, such as automatic emergency braking systems and collision warning systems, which detect the relative speed of nearby vehicles.

Geophysics: Application in seismic techniques to study the internal structure of the Earth. Analysis of seismic waves using the Doppler Effect can help map discontinuities and underground structures.

Wireless and mobile network technology: In improving mobile communication and reducing interference. Doppler compensation is crucial in communication systems that operate at high speeds, such as those used in high-speed trains and vehicle-to-satellite communications.

Oceanography: Using Doppler sonar, scientists can measure underwater ocean currents, which is crucial for understanding ocean dynamics, global climate, and circulation patterns.

If you liked it, please share. And if you know of any other application of the Doppler effect, please leave it in the comments :)