(WVS) Doppler Effect Lesson

Doppler Effect

Have you noticed that when an emergency vehicle with its siren blaring passes you that the tone that you hear changes in pitch? This is an example of what is called the Doppler shift, and it is an effect that is associated with any wave phenomena. It is the same effect that Hubble Space Telescope used to measure the velocities of distant galaxies.

Watch the video below for an explanation of what the Doppler Effect is and see how it affects both sound and light waves:

In summary, this is how it works:

First, consider a case where the fire truck is at rest in the station driveway waiting for the firemen to board the fire truck. The siren is already on, and a listener some distance away to the right will perceive the siren at the same frequency as which it is emitted. Note that another stationary person on the left side of the truck would hear the same tone also.

Now consider how this situation changes when the truck is moving towards the stationary observer with a constant velocity, v.

The frequency of the fire engine's siren as heard by a person on the fire truck has not changed! However, the waves in the direction of the truck bunch up as the fire truck is catching up to its own sound waves. This means that the pressure variations impinge upon the eardrum of the stationary observer at an increased frequency. The stationary observer to the right, therefore, perceives a higher pitch. Notice also that the waves traveling towards the rear of the fire truck seem to spread out as the siren is moving away from its own sound as it travels to the right. This would cause a stationary observer to the left of the truck to perceive a decrease in the frequency which would result in a lower pitch sound of the siren.

Without much additional thought, as I know you have been thinking for a while now, you could easily convince yourself that the Doppler shift will occur under any of the following circumstances:

• The source is approaching a stationary observer.

• The observer is approaching a stationary source.

• The source and the observer are moving toward one another.

• The source is moving away from a stationary observer.

• The observer is moving away from a stationary source.

• The source and the observer are both moving away from each other.

• The source and the observer are moving in the same direction at different speeds.

You should also be able to easily convince yourself that the shift will yield an increase in the perceived frequency whenever the source and the observer are approaching one another, and a decrease in the perceived frequency whenever the source and the observer are moving away from each other.

How does this affect the spectra of distant objects in the Universe, you might ask? Do light waves Doppler shift as well as sound waves? Yes, light waves can be shifted up in frequency or down depending on your relative motion. In fact, if your recession velocity is great enough away from a visible light source, you could in theory warm yourself as you would be able to shift to the infrared or heat area of the electromagnetic spectrum. Now take a moment to consider the diagram that follows:

You will recognize it as the same as the fire truck approaching the stationary observer except now the source is emitting light instead of sound. Notice that the right region where a perceived increase in the frequency is noticed is referred to as "blueshifted", and the region which would appear to be of a lower frequency to an observer on the left is referred to as "redshifted."

At normal speeds, when an object generates sound, the sound travels away from the object and people hear the same sound that was generated by the object. In some unusual cases, like this airplane, the object can travel as fast or faster than the sounds it makes. The sound wave moves along with this airplane; as the airplane continues to generate more sound, this sound is added to the old sound. When this extra large sound wavefront arrives, the sound is much louder than the sound that was originally generated. When this happens with airplanes, the sound is called a "sonic boom."

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