# Doppler effect

1. Jul 4, 2015

### 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

2. Jul 4, 2015

### Staff: Mentor

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.

3. Jul 4, 2015

### Dr. Courtney

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.

4. Jul 4, 2015

### Amr Elsayed

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 ?

5. Jul 4, 2015

### nasu

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

6. Jul 4, 2015

### Staff: Mentor

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 travelling 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.

7. Jul 5, 2015

### bcrowell

Staff Emeritus
8. Jul 5, 2015

### Amr Elsayed

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