Explaining the Doppler Effect on Photons: Consistency with Wave Theory

In summary, when a light source is stationary with respect to an observer, the energy loss detected by the observer is E = nhf. However, when the light source is moving with respect to the observer, the apparent frequency detected by the observer is different, leading to a different energy loss of E = nhv. This can be explained by the relativistic contractions and dilations in the reference frame. This demonstrates that the energy of a particle is dependent on the reference frame in which it is measured, consistent with the wave theory of light.
  • #1
Pandemonium
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Consider a situation in which a light source is stationary with respect to an observer, A. This source emits n photons of frequency f, each of energy E = hf, towards A. Hence, A will be able to detect the energy loss in the source, which is E = nhf.

Now, this light source is moving with respect to A, and emits n photons of frequency f just like before. However, due to the Doppler Effect, the apparent frequency in which A detects is different, say v. As a result, the energy loss in the source detected at A will be E = nhv, and not E = nhf, which should be the actual energy loss.

How can this be explained? And is this consistent with the wave theory of light?
 
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  • #2
You have done this all in a Newtonian framework. But because photons move at c you have to do it with the relativistic contractions/dilations. When you do this (there are many sites on the web that do the calculations, or you could read Spacetime Physics by Wheeler and Thorne), it all works out. Relativity provides a consistent theory of the doppler effect.
 
  • #3
Well, what you are pointing out is simply that the energy a particle has depends upon the reference frame in which it is measured. There is no "actual" energy loss of the emitter just as there is no "actual" time that an event occurs, but rather these quantities must be related to a particular reference frame.

dhris
 
  • #4
Thanks for the help, guys!
 

1. What is the Doppler Effect on Photons?

The Doppler Effect on Photons is a phenomenon in which the frequency and wavelength of light waves are affected by the relative motion between the source of light and the observer. This results in a change in the perceived color of light.

2. How does the Doppler Effect on Photons work?

As an object emitting light moves towards an observer, the distance between successive wave crests decreases, causing an increase in frequency and a shift towards the blue end of the color spectrum. Conversely, when the object moves away from the observer, the distance between wave crests increases, resulting in a decrease in frequency and a shift towards the red end of the color spectrum.

3. What is the difference between the Doppler Effect on Photons and the Doppler Effect on Sound Waves?

The main difference between the two is that the Doppler Effect on Photons involves changes in the frequency and wavelength of light waves, while the Doppler Effect on Sound Waves involves changes in the frequency and wavelength of sound waves. Additionally, the Doppler Effect on Photons is applicable to all electromagnetic waves, while the Doppler Effect on Sound Waves is specific to audible sound waves.

4. How is the Doppler Effect on Photons used in astronomy?

The Doppler Effect on Photons is used in astronomy to determine the relative motion and distance of objects in space. By observing the shift in the frequency of light emitted from stars and galaxies, scientists can calculate their velocity and direction of motion.

5. Can the Doppler Effect on Photons be observed in everyday life?

Yes, the Doppler Effect on Photons can be observed in everyday life. For example, the sound of an ambulance siren changing as it approaches and then moves away is due to the Doppler Effect on Sound Waves. The same effect can be observed with the changing pitch of a passing train or car horn.

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