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# Doppler Effect Applications for Electromagnetic Waves

by Ron Kurtus (updated 6 January 2022)

The Doppler Effect is the change in waveform frequency and wavelength caused by a moving source or observer. There are various applications of the Doppler Effect for electromagnetic waves.

In Astronomy, the movement of a distant light source—such as a star or galaxy—can be measured by the Doppler Effect. It is often called red-shift or blue-shift, depending on the direction of the motion of the source.

Communication signals from satellites circling the Earth can have a Doppler shift relative to a ground station. The speed of a moving vehicle can be measured using Doppler radar.

Questions you may have include:

• How is the Doppler Effect used in Astronomy?
• How is it used in communication with satellites?
• How does Doppler radar work?

This lesson will answer those questions. Useful tool: Units Conversion

## Radio waves: Compensation in satellites

When communication signals are sent from the Earth to a fast moving satellite, the received frequency is shifted due to the Doppler Effect. Likewise, the signals received by the Earth station are shifted due to the motion of the satellite.

A method called Dynamic Doppler compensation changes the frequency of the signal multiple times during transmission to help keep the signal frequency constant.

Radar devices emit electromagnetic waves—usually in the microwave region—and receive those waves when they are reflected off an object. The time it takes for the waves to make the round trip is measured, thus determining the distance.

In Doppler radar, measurement of the velocity of the object is achieve by comparing the original wave frequency with the frequency of the reflected wave, which was changed due to the Doppler Effect off the moving object. It is often used by the police to monitor the speed of automobiles.

Note: What is called "Doppler radar" in television weather reports is really a highly specialized for of Doppler radar that includes information about the motion of precipitation. It uses a special pulse-Doppler technique.

## Visible light: Used in Astronomy

The motion of stars and galaxies toward or away from the Earth can be measured using the Doppler Effect with a spectrometer.

A spectrometer is a device that separates the input colors and spreads them out in a display. Since the incoming light is made up of distinct bands of color, according the chemical composition of the star, the spectrometer will show a shift in the color from what it would be if the source was not moving.

A distant source of light that is moving toward the observer on Earth will cause the observed wavelength of the light to shift toward higher frequencies or shorter wavelengths. This is called the "blue-shift" since visible light shifts toward blue colors.

When the distant source is moving away from Earth, the light will be shifted toward the red colors, according to the velocity of the source. This is call the "red-shift" of the light.

## Summary

The Doppler Effect for electromagnetic waves is used in various applications. In Astronomy, the movement of star or galaxy can be measured by the Doppler Effect, using a spectrometer. It is often called red-shift or blue-shift, depending on the direction of the motion of the source.

Communication signals from satellites circling the Earth can have a Doppler shift relative to a ground station. Also, the speed of a moving vehicle can be measured using Doppler radar.

Be clever in applying physical principles

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