Doppler Effect

Introduction 

The apparent variation in frequency between the frequency at which sound or light waves leave a source and the frequency at which they reach an observer, produced by relative motion between the observer and the wave source, is known as the Doppler effect. The Doppler effect was first described in 1842 by Austrian physicist Christian Doppler. The Doppler effect is a key feature of modern universe theories and is used to examine the velocity of stars and to seek double stars. ​​The Doppler effect can be used to study any type of waves, like- water waves, sound waves, or lightwaves, etc. But most commonly it is used for studying sound waves.

Uses of Doppler effect

The Doppler effect has several real-life applications. Here are some of the instances where Doppler effect is used:

In astronomy – Astronomers who use information about the shift in frequency of electromagnetic waves produced by moving stars in our galaxy and beyond to infer information about those stars and galaxies. The theory that the universe is expanding is based on measurements of electromagnetic waves released by distant galaxies. Furthermore, the Doppler effect can be used to determine specific information about stars within galaxies.

In light – The apparent shift in frequency of light viewed by the observer owing to relative motion between the source of light and the observer is known as the Doppler effect of light. Light does not require a medium, thus the Doppler shift for light traveling in a vacuum is determined only by the observer and source’s relative speeds.

In vibration – The Laser Doppler Vibrometer (LDV) is a non-contact vibration measurement device. The LDV emits a laser beam that is directed towards the surface of interest, and the frequency and amplitude of the laser beam frequency may be read from the Doppler effect of the laser beam frequency due to movement on the surface.

In radar  – The Doppler effect is used by some radars to calculate the velocity of target objects. A radar beam is directed at a moving target, such as a vehicle. The police employ radar to detect high-speed automobiles. Shooting a moving vehicle with a radar beam allows the police to correctly compute the vehicle’s velocity and indicate how quickly it is driving.

In Sirens – Everybody is familiar with sirens. The sliding pitch of a moving siren, whether it’s an ambulance, a police siren, or a fire truck bell, is something we’re all familiar with. The Doppler effect causes this apparent change in pitch, as stated above. In reality, though, the observation is not so evident. We hear an emergency vehicle’s siren gradually increase as it approaches us and gradually decrease as it recedes. It’s due to the wave’s angular resolution. To put it another way, we are not directly in the path of an approaching or receding source. We examine the radial component of the source’s velocity since the wavefront hits us at an angle.

In Echocardiogram – An echocardiogram is a test that determines the direction and speed of blood flow. The Doppler effect can be used to determine the velocity and direction of moving blood using ultrasonic waves. We may measure the velocity of blood cells in echocardiography by measuring the size of the frequency shift between the transmitted and received signal, just as we can determine the speed of a moving vehicle with a radar gun. Depending on whether the Doppler shift is positive or negative, we may also detect the direction of the flowing blood.

In satellites – When tracking satellites, the Doppler effect is commonly observed in the frequency of signals received by ground receiving stations. A combination of the satellite’s route, its orbit around a planet, Earth’s revolution around the sun, and Earth’s daily rotation on its axis may cause the rising or decreasing distance between the satellite and the ground station. The received signal will have a positive frequency bias due to a spacecraft nearing Earth. However, as the satellite passes Earth, the received Doppler bias will become zero, and as the satellite moves away from Earth, it will become negative. The relative velocity between the transmitter and receiver, or the relative phase velocity, which is the relative velocity modified by the propagation medium, is proportional to the frequency shift generated by the Doppler effect. Ground stations on Earth monitor and track satellite navigation by taking all of these factors into consideration.

Doppler effect formula

The Doppler effect formula can be used to compute the velocity of the source and observer, as well as the original and observed frequencies of the sound waves. While there is only one Doppler effect formula, it changes based on the observer’s or sound source’s velocities in different scenarios. This is the Doppler effect formula:

f’=(v + vo)(v – vs) f

Here, f’ = observed frequency

f = actual frequency

v = velocity of sound wave

vo = velocity f the observer

vs  = velocity of the source

Doppler effect limitations

The Doppler effect has certain limitations. To watch the effect of Doppler, it is necessary that the velocity of the source, observer, and medium all be smaller than the velocity of sound. If their velocity exceeds the speed of sound, the wave velocity graph becomes distorted, and frightening waves result. Its example is a jet plane; when the speed of the jet plane exceeds the speed of sound, the Doppler effect is absent.

In electrocardiograms, one of the limitations is that the ultrasonic beam should be as parallel to the blood flow as feasible, which is one of the restrictions. Velocity measurements are used to check cardiac valve regions and function, as well as any aberrant communications between the left and right sides of the heart, blood leakage through the valves (valvular regurgitation), and cardiac output computation. To improve velocity or other flow-related medical measurements, contrast-enhanced ultrasonography with a gas-filled microbubble contrast medium can be employed.

Conclusion

So we have seen that the Doppler effect is a common occurrence in all forms of waves, including sound waves, light waves, and even water waves. The Doppler effect has been much explored by scientists to produce some incredible discoveries. 

To measure velocities in a fluid flow, instruments like the laser Doppler velocimeter (LDV) and the Acoustic Doppler Velocimeter (ADV) have been created. The LDV and ADV both emit a light or acoustic beam and measure the Doppler shift in wavelengths of reflections from moving particles. This technology enables non-intrusive, high-precision, and high-frequency flow measurements.