Doppler effect for light wave

In summary: Doppler effect comes from the fact that the observer is not at the same distance from each wave crest, when the source is moving. So the wave crests are not arriving at the observer's eye at regular intervals, which is what makes the frequency different.The relativistic Doppler effect formula takes into account the fact that the source could be moving, as well as the observer, so there are actually two different velocities involved.
  • #1
loup
36
0
How to prove the doppler effect for light wave?

Isn't that the speed of light is constant for both the observer and source?
If the wavelength for both the source and observer is the same, then the frequency is the same too, so there is no doppler effect?

If the wavelength is different, but if we use Lorren's transfer, the new wave obtained will be the same if the observe is moving towards or leaving the source!

How to deal with this?
 
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  • #2
the speed is the same but the frequency's are different
 
  • #3
cragar said:
the speed is the same but the frequency's are different

In sound wave, the sound frequency depends on speed, why not in this case?
 
  • #4
i guess the same reason you don’t just add velocities like you do two moving cars
with light you can't add the velocities so the light is always perceived to the observer at c and all frequency's travel at c. i guess when you travel towards the light source you see a blue shift the distance between the crest's is shorter as your eye perceives it but the time between the crest's is shorter so the speed still comes out c .
Dont know if this is 100% accurate.
 
  • #5
How come the wavelength of light is shortened after it falls from the top of a 3 story building to the basement? The famous Mossbauer Effect experiment at Harvard by Pound and Rebka in 1959 showed that the 14 keV photons from an iron-57 source gained energy (wavelength shortened) when they fell from the top of the physics building.
 
  • #7
jtbell said:

Thanks for the site, but a formula is a formulae. Does anyone explain it a little bit more ?
 
  • #8
  • #9
Are there any substantial experiments with visible light, and the doppler effect, that don't rely on large celestial bodies? Or the travel of light through space?
 
  • #11
What about my questions?

Proof for sound effect I clearly understand. But light cannot be treated as relative velocity, it is absolute, isn't it?
 
  • #12
The Doppler effect is just a consequence of the fact that if an object emitting regular signals (or peaks of a continuous wave) is in motion relative to you, then each signal (or peak) has a different distance to travel to reach your eyes...the relativistic Doppler effect also factors in time dilation, but that's the only difference. For example, suppose a clock is traveling away from me at 0.6c, and it's programmed to send out a flash every 20 seconds in its own rest frame. In my frame, because of time dilation the clock is slowed down by a factor of [tex]1/\sqrt{1 - 0.6^2}[/tex] = 1.25, so it only flashes every 1.25*20 = 25 seconds in my frame. But that doesn't mean I see the flashes every 25 seconds, the gap between my seeing flashes is longer since each flash happens at a greater distance. For example, suppose one flash is emitted when the clock is at a distance of 10 light-seconds from me, at time t=50 seconds in my frame. Because we assume the light travels at c in my frame, if the flash happens 10 light-seconds away the flash will take 10 seconds to reach me, arriving at my eyes at t=60 seconds. Then, 25 seconds after t=50, at t=75, the clock emits another flash. But since it was moving away from me at 0.6c that whole time, it's increased its distance from me by 0.6*25 = 15 light-seconds from the distance it was at the first flash (10 light-seconds away), so it's now at a distance of 10 + 15 = 25 light-seconds from me, so again assuming the light travels at c in my frame, the light will take 25 seconds to travel from the clock to my eyes, and since this second flash happens at t=75 in my frame, that means I'll see it at t=100 seconds. So, to sum up, the clock flashes every 20 seconds in its own rest frame, and once every 25 seconds in my frame due to time dilation, but I see the first flash at t=60 seconds and the second at t=100 seconds, a separation of 40 seconds. This means the frequency that I see the flashes (1 every 40 seconds) is half that of the frequency the clock emits flashes in its own frame (1 every 20 seconds), which is exactly what you predict from the relativistic Doppler equation if you plug in v=-0.6c (negative because the clock is moving away from me): [tex]\sqrt{\frac{1 - 0.6^2}{1 + 0.6^2}} = \sqrt{0.25} = 0.5[/tex]. And you can see from the italics above that I specifically assumed the light from each flash traveled at exactly c between the clock and my eyes.
 
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  • #13
loup said:
How to prove the doppler effect for light wave?

See for example the link in post #8.

Isn't that the speed of light is constant for both the observer and source?

Yes.

If the wavelength for both the source and observer is the same,

No, it isn't, if you mean "wavelength in the source's rest frame" and "wavelength in the observer's rest frame."

then the frequency is the same too, so there is no doppler effect?

The speed of the wave is the same for both observers, so if the wavelength is different, then the frequency must be different, also.

The relativistic Doppler effect actually involves three different frequencies, and you need to be clear about which one you are talking about:

1. The frequency of the source, in its own rest frame. An observer riding along with the source would see this frequency. I call it the proper frequency of the source.

2. The frequency of the (moving) source, in the observer's rest frame. This is different from the proper frequency because of time dilation.

3. The frequency of the wave emitted by the source, as received by the observer. This is different from #2 for exactly the same reason as in the classical Doppler effect: the source is moving, so successive "peaks" of the wave are emitted from different locations in the observer's reference frame. If the source is moving towards the observer, each successive peak needs to travel a shorter distance to reach the observer. It arrives at the observer earlier than if the source had remained stationary, which increases the frequency at the observer. Similarly, if the source is moving away from the observer, each successive peak needs to travel a larger distance, so it arrives at the observer later than if the source had remained stationary.
 

1. What is the Doppler effect for light wave?

The Doppler effect for light wave is a phenomenon where the perceived frequency of light changes as the source of light moves towards or away from an observer. This results in a shift in the color of the light, known as the redshift or blueshift.

2. How does the Doppler effect for light wave work?

The Doppler effect for light wave works by changing the wavelength of light as the source moves towards or away from an observer. When the source moves towards the observer, the wavelength of the light appears shorter, resulting in a blue shift. Conversely, when the source moves away from the observer, the wavelength appears longer, resulting in a redshift.

3. What causes the Doppler effect for light wave?

The Doppler effect for light wave is caused by the relative motion of the source of light and the observer. This can be observed in various situations, such as when a star or galaxy is moving towards or away from Earth, or when a vehicle with lights is moving past a stationary observer.

4. How is the Doppler effect for light wave different from the Doppler effect for sound wave?

The main difference between the Doppler effect for light wave and sound wave is that light waves travel much faster than sound waves. This means that the Doppler effect for light wave is more significant in astronomical observations, while the Doppler effect for sound wave is more commonly observed in everyday situations.

5. What are some real-world applications of the Doppler effect for light wave?

The Doppler effect for light wave has many real-world applications, such as in astronomy for measuring the speed and direction of stars and galaxies. It is also used in medical imaging techniques like Doppler ultrasound to measure blood flow and in radar technology to detect the speed and location of moving objects.

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