The Doppler Effect

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by AnnaB1297
Last updated 6 years ago


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The Doppler Effect

When a policeman points a radar gun at you, he can effectively determine your speed thanks to the Doppler effect. - A radar gun sends out radio waves at a pre-established frequency. - These waves travel from the police car to the civilian car being clocked and are then reflected back to the radar gun. - Once the radar gun receives the radio waves back, it can determine the frequency of the waves. - Since the police car is stationary and the civilian car is moving toward the police car, the frequency of the radio waves received by the radar will be greater than the frequency the radar originally sent out. - Since frequency and velocity are directly proportional, the higher a car's velocity, or speed, the higher the frequency of the radio waves received by the radar gun. The greater the change in frequency in radar waves emitted and reflected, the faster the car is traveling. - Using the equation above, the velocity of the source of the sound can be determined because the emitted and observed (reflected) frequencies, as well as the velocities of the sound traveling through the air and the observer, are known. - Using this information, police can then determine whether or not a car is speeding.

Waves in Water

First predicted by Austrian mathematician, Chrisitan Johann Doppler, in 1842.Applies to all waves, including light waves, which are transverse waves, and sound waves, which are longitudinal waves. Definition/Explanation:The Doppler effect occurs when there is a moving source of sound that emits longitudinal sound waves. Those whom the sound approaches hear a higher observed frequency (pitch) because the approaching sound waves are compressed and more waves hit the observer over a shorter period of time than if the source of the sound was stationary. Those whom the sound recedes from hear a lower observed frequency (pitch) because the waves are spread out and fewer waves hit the observer over a longer period of time than if the object was stationary. Although the actual frequency remains constant, the observed frequency differs depending on whether the source is approaching or receding from an observer.

- An observer will notice the constant wavelength of waves being emitted from a stationary source, regardless on whether or not the observer is standing in front of or behind the source. - When the source begins to move, it will catch up to the waves emitted from it that are traveling in the same direction as the source. - An observer located in front of the moving source will notice that the waves look much more compressed and appear to be moving at a much higher frequency, while an observer located behind the moving source will observe waves moving at a much slower frequency.


Astronomers utilize the Doppler effect to measure the speed of stars, planets, and comets in much the same way as a policeman's radar gun measures the speed of a car. The frequencies of the waves emitted by NASA technology hit and bounce off of a targeted object in space and return back to NASA headquarters. The received frequencies tell astronomers whether or not the object is moving towards or away from earth (condensed vs. spread out waves) and how fast the object is moving (reflected velocity of the source). Astronomers also use the Doppler effect to locate stars in order to find the Milky Way. This is done in the same manner as described above. Finally, astronomers have determined, by utilizing the Doppler effect, that the universe is expanding and the galaxies are actually moving away from each other because the frequencies of light waves are lengthening. The big bang is now even closer to being proven.

Doppler Radar

1. Weather stations emit large waves of predetermined frequencies into the sky towards the clouds. 2. The waves hit precipitation and other weather conditions and are reflected back toward the station. 3. Meteorologists can then read the received frequencies and determine if a storm or weather condition is moving toward or away from the weather station. If a storm is moving toward the weather station, the received wave frequencies will be higher than the original frequencies. If a storm is moving away from the weather station the received wave frequencies will be lower than the original frequencies emitted by the station's radar.

The Doppler Effect

Police Radar

Doppler Effect



Submarines use the doppler effect to detect undersea objects such as fish and mines. Submarine sound waves bounce off of the undersea objects and reflect back to the submarine. The radar equipment can then determine whether or not the object is stationary, moving toward the submarine, or moving away from the submarine. It can also determine how fast the object is traveling.

As a car or airplane passes an observer, the sound the observer's ear perceives is emitted from the the vehicle changes depending on the direction that the vehicle is traveling in. When the car or plane is approaching you, you hear a higher frequency because the vehicle is moving towards you and its sound waves are compressed in your direction. Once the car or plane passes you and begins moving away, the perceived frequency of the sound decreases and the pitch sounds lower because the sound waves stretch out as the car or plane gets farther away. This is why you hear a "VROoom" when a car passes you.

Moving Vehicles

By: Anna Baker

The Milky Way


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