Explanation of the Doppler Effect - Succeed in Physical Science. Also refer to velocity, speed, wave, waveform, frequency, wavelength, sound, pitch, light, color, sonar, doppler radar, physics, Ron Kurtus, School for Champions. Copyright © Restrictions
Doppler Effect
by Ron Kurtus (15 March 2008)
When the source of a waveform—such as sound or light—is moving, the detected frequency or wavelength is different than what was emitted. This change is called the Doppler effect. The wavelength appears shorter when the source is moving toward you and appears longer when the source is moving away. You may have heard the pitch of the siren in an ambulance sound higher as the vehicle moved toward you and then get lower as it moved away. The Doppler effect is also used in astronomy to measure the motion of galaxies as they move away from the Earth. This is called the red shift.
Questions you may have include:
- Why does the Doppler effect happen?
- How does it affect sound?
- How it the effect used in astronomy?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
Useful tools: Metric-English Conversion | Scientific Calculator.
Moving source
The source of a waveform—such as sound or light—sends out a wave at a regular rate or frequency. But if that source is moving, it starts to catch up to waves in the direction of motion, as well as pulls away from the waves opposite the motion. The velocity of the waveform does not change, but the wavelength and frequency does change due to the motion of the source.
This causes the wavelength in the direction of motion to be shorter and their frequency higher. Likewise, the waves moving in the direction opposite the motion of the source will have longer wavelengths and lower frequencies.
Doppler effect caused by a moving source
The illustration shows how when the source of the waves moves to the left, the frequency of the waves in that direction is higher. The wavelength in the opposite direction is lower.
If the source of the waveform is stationary and the wave reflects off a moving object, it is as if the object was a moving source. The wavelength and frequency will change, according to the Doppler effect.
Sound waves
The Doppler effect is experienced when a source is moving and when the sound is reflected off a moving object.
(See Doppler Effect Equations for Sound for information on calculating the effect.)
Note that the velocity of the source or moving object must be less than the speed of sound. A speeds equal to or greater than the speed of sound, a somewhat different effect takes place.
(See Traveling Faster than Sound for more information.)
Source is moving
When you hear the sound of an object moving toward you—such as a siren from and emergency vehicle—the pitch of that sound is higher than if the vehicle was standing still. Likewise, as the vehicle moves away from you, the siren would have a lower pitch.
Reflected off moving object
If a stationary source sends out sound waves and they are reflected back off a moving object, the sound waves heard at the source will be a different pitch, according to the direction of the object. This is as if the moving object was sending out the sound waves. While sonar uses sound waves to measure the distance to an object, doppler sonar also measures the speed of the object.
Electromagnetic waves
Astronomers use the Doppler effect to measure the velocities of the galaxies with respect to the Earth. Also, Doppler radar uses electromagnetic waves to measure the speed of vehicles and storms.
(See Doppler Effect Equations for Light for information on calculating the effect.)
Measure velocity of galaxy
Most stars and galaxies have a high concentration of Hydrogen, which in turn has a specific spectrum of colors when it is extremely hot, such as on our Sun and in other stars. In measuring the wavelengths of the light from distant galaxies, it appears that the spectrum shifted slightly toward the red. This means that the wavelengths are longer than they should normally be, indicating the Doppler effect caused by a source of light moving away from the Earth.
This is called the red-shift. From the measurements, it can be calculated how fast the galaxies are moving away from us. It also shows that the Universe is expanding.
Galaxies and stars that are moving toward our Solar System would send out shorter wavelengths as a blue-shift.
Doppler radar
Doppler radar sends out electromagnetic waves—usually in the microwave region—to calculate the velocity of moving objects by measuring the Doppler effect caused by reflection off the moving object.
Police use radar devices to send out an electromagnetic beam of a certain wavelength. When that waveform is reflected off a moving automobile, the motion causes the reflected wavelength to increase, thus allowing the doppler radar device to calculate the speed of the vehicle.
Summary
The Doppler effect is the perceived change in frequency or wavelength caused when the source of the waveform is moving. The wavelength appears shorter when the source is moving toward you and longer when the source is moving away. You can have heard the pitch of a siren sound higher as the vehicle moved toward you and then get lower as it moved away. The Doppler effect is also used in astronomy to measure the motion of galaxies as they move away from the Earth.
Move with the flow
Resources
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Mini-quiz to check your understanding
1. Why doesn't the speed of the waveform change when the source is moving?
2. When would you hear the actual pitch of the siren of a moving ambulance?
3. How can astronomers tell which galaxies are moving away the fastest?
If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.
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