# Understanding the Doppler Effect: Differences Between Sound and Light

• Amr Elsayed
In summary, the Doppler effect is caused by the relative motion between a source and an observer. The basic reason for the effect is the same for both sound and light, but the details of how it is calculated differ. For sound, the speed of the pulse across the changing distance is relative to air, while for light, the speed is always constant at c relative to both the source and receiver. Time dilation applies in both cases, but is generally insignificant for sound. However, for light, it must be taken into account in the calculation of the Doppler effect. The rate of emission and receiving of light can appear different depending on the frame of reference, but this is due to the effects of time dilation and does not affect the overall
Amr Elsayed
Hi,
Does Doppler effect differs in reason from sound to light ? I mean is the Doppler effect of sound just because changing velocity of sound? In the case of light it's because of merely time dilation

Amr Elsayed said:
Hi,
Does Doppler effect differs in reason from sound to light ? I mean is the Doppler effect of sound just because changing velocity ? In the case of light it's because of merely time dilation

The basic reason for the Doppler effect is the same in both cases - if the source and the destination are moving relative to one another, the distance between them and hence the travel time for successive pulses will be changing. The details of how this is calculated are different: for sound waves the speed of the pulse across the changing distance is relative to air, which may be moving relative to either or both of the source and the receiver; for light, the speed of the pulse is c relative to both source and receiver. Time dilation applies in both cases, but the effect is generally insignificant when we're working with sound.

Amr Elsayed
There are different ways to think about it. I think about the difference being due to the fact that sound requires a medium and light does not.

The speed of the source and the observer relative to the medium are important in the doppler effect in sound.

For light, there is no medium, so the only relative velocity is the source and observer relative to each other.

In either case, you can think about the number of wave crests passing the observer and compute the proper shift in frequency.

Amr Elsayed
Nugatory said:
Time dilation applies in both cases, but the effect is generally insignificant when we're working with sound.
Okay I know that time dilation applies in both cases, but because the speed of sound is changing unlike C, the change in frequency of sound will not be just about time dilation ?? I mean increasing speed of sound would necessarily associate a change in the frequency ?

There is no change in speed of sound associated with Doppler effect.

Amr Elsayed said:
Okay I know that time dilation applies in both cases, but because the speed of sound is changing unlike C, the change in frequency of sound will not be just about time dilation ?? I mean increasing speed of sound would necessarily associate a change in the frequency ?

Don't ask me, calculate it for yourself.

You have a source that emits a 1 Hz sound wave and also flashes a light once per second. These signals are traveling to a receiver, and you can calculate the travel time from source to receiver for each individual flash of light and pulse of sound from the distance between the two (which changes with their motion) and the speed with which the signal covers that distance. First do this calculation assuming that all the speeds are low enough that time dilation can be ignored... and only after you've worked through that case are you ready to consider the relativistic Doppler effect that also considers the effects of time dilation caused by the relative motion of source and receiver.

Amr Elsayed
bcrowell said:
No, this is not true. We discussed this in the other thread you started: https://www.physicsforums.com/threads/rate-of-emission.821970/ .
Okay I now get how relative velocity affects rate of receiving light according to the moving body. But we think that time dilation will increase the rate of receiving light by the moving body when we relate the receiving of pulses to a specific time on the clock with the moving body which appears to be slow for me. But on the other hand if we relate the shooting of pulses to specific times on the fixed clock on the source, shouldn't I see the rate of emission smaller ? I know it's not, but why not ? I discussed this question in the other thread by the way

## What is the Doppler Effect?

The Doppler Effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the source of the wave. It is commonly observed in sound waves, such as the change in pitch of a siren as it passes by, and in light waves, such as the shift in color of a star as it moves away from us.

## How does the Doppler Effect differ between sound and light?

The main difference between the Doppler Effect in sound and light is the medium in which the waves travel. Sound waves require a medium, such as air or water, to travel through, while light waves can travel through a vacuum. This difference in medium affects the speed at which the waves travel, resulting in different observed effects.

## What causes the Doppler Effect?

The Doppler Effect is caused by the relative motion between the source of the wave and the observer. If the source is moving towards the observer, the waves will be compressed, resulting in a higher frequency (shorter wavelength) and a higher pitch (in sound waves). If the source is moving away from the observer, the waves will be stretched, resulting in a lower frequency (longer wavelength) and a lower pitch.

## How is the Doppler Effect used in science?

The Doppler Effect is used in various fields of science, including astronomy, meteorology, and seismology. In astronomy, it is used to determine the movement and distance of celestial objects. In meteorology, it is used to track the movement of storms. In seismology, it is used to analyze seismic waves and determine the location and magnitude of earthquakes.

## What are some real-life examples of the Doppler Effect?

Some common examples of the Doppler Effect in everyday life include the change in pitch of a car horn or ambulance siren as it passes by, the shift in sound of a train as it approaches and then moves away, and the change in frequency of a police car siren as it moves towards and then away from us. In astronomy, the redshift of distant galaxies is a result of the Doppler Effect, as the light waves are stretched due to the expansion of the universe.

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