# Doppler effect for light - how can it be explained ?

1. Sep 19, 2010

### fsoica

Did u ever try to explain Doppler effect for light in depth, at the photon level ?

I'm no scientist.

I'm trying to find a visual or mental representation of the doppler effect for electromagnetic waves.

There is no medium to vibrate. No way I can understand the phenomenon by making a parallel with sound waves, water waves or some ball throwing analogy, like in all the examples available online.

I cannot understand where does the change in frequency of the emitted light come from ? It is not supposed to be a change in photon energy also ? If so, how this loss/gain of energy occurs ?

I have a lamp. It emits green light. It starts to "run away" from me, faster and faster. According to Doppler effect theory, the light that I see becomes redder and redder, if I could say so. I find this impossible to explain.

The only webpage that contains an explanation relatively close to what I'm searching (or I think I'm searchin') is here (alternative theory website, but pls. bear with the guy, it sounds quite right):

Crackpot link deleted. Please read the rules of this site.

Is there something wrong with the above view about the phenomenon ?

Am I missing something?
Is there a possible way of popularizing the Doppler effect for light, in a different manner from the sound wave analogy ?

Last edited by a moderator: Sep 20, 2010
2. Sep 19, 2010

### Dr Lots-o'watts

From the link :

"Feeling frustrated? Well that must mean that you are just inferior to me, or perhaps it’s because the whole concept is a big ball of LSD induced illogic? Let’s just not apply occam’s razor to it because I prefer being superior to you"

What a load of bull (the link). He's basically running the apparent paradox that lead to Michel-Morley's experiment, which itself lead to the need for SR.

Last edited: Sep 19, 2010
3. Sep 19, 2010

### fsoica

Forget about the link and its argument.
Can you pls. explain in plain english the Dopppler effect for light ?

4. Sep 19, 2010

### Drakkith

Staff Emeritus
The frequency of light is simply the energy of it. (IE the greater the energy of the photon, the greater its frequency)

If im running away from light, then when that light hits me, it doesnt impart as much energy into me since i'm already moving, hence it will be redshifted. And its the opposite case for me heading towards light.

The photon itself is merely an electromagnetic wave that doesnt require a physical substance to vibrate in, not air, not water, nothing. The wave itself is vibrating. Think of a magnet. It doesnt require a medium to transfer its force. It works equally as well in a vacuum as it does in air.

Also, in regards to the frame of reference stuff, all i can say is how i think of it: Light is already traveling at c. It always travels at c. If your lamp is moving away from you, then because light has to travel at c, it has to "expend" energy to move at c in your direction, hence redshifting it. If the lamp is moving towards you, it gets a "boost" of energy from the velocity of the lamp, blueshifting it. Instead of actually moving faster or slower, the light takes the energy of the observer and the emitter and adds it into itself when it "enters your frame of reference".

Forgive me, im sure ive broken a dozen rules on science, light, and relativity, but thats kind of how i picture it.

5. Sep 19, 2010

### Staff: Mentor

fsoica, the link is not appropriate for this forum at all and should be removed. This forum is not a spot for advertising for fringe and crank websites.

Regarding the Doppler effect. It is very easy. Simply write the equation for the wave as a function of time and position and the equation for the observer's position as a function of time. Then it is just a matter of substitution to find the observed frequency.

6. Sep 19, 2010

### Entropee

Drakkith explained it very nicely.

7. Sep 19, 2010

### Drakkith

Staff Emeritus
Is there something wrong with posting a link to a website that causes you to question something in science? It seems obvious to me that this isnt a deliberate attempt to advertise the site or anything.

8. Sep 19, 2010

### fsoica

I have not intended even for a second to advertise anything. I posted that particular link only because after a few hours of searching on the web some text trying to shed some light on the Doppler effect for ... light (for the layman, the one who has no idea of equations and so on), the only site I came up with was that particular alternative theory website.

9. Sep 20, 2010

### rcgldr

Another approach would be to note that the speed of light is constant regardless of the frame of reference, and that frequency and wavelength are constant relative to a specific light source. If a light source and observer are moving towards each other, then the distance between light source and observer decreases with each cycle of a wave, resulting in observed shorter wavelength and higher frequency. If the light source and observer are moving away from each other, the result is an observed longer wavelength and lower frequency.

Wiki has an article that also takes special relativity into account:

http://en.wikipedia.org/wiki/Relativistic_Doppler_effect

Last edited: Sep 20, 2010
10. Sep 20, 2010

### alxm

The classic lament of the crackpot: "Censorship! I'm being supressed! Science doesn't tolerate dissent!"

Science is built around questioning stuff and dissenting. But we don't have to tolerate crackpots, because they aren't actually interested in our answers to their questions. They're interested in self-aggrandizement and their own Great Theory of Everything. They refuse to learn, no matter how many times you attempt to correct them. They refuse to learn, refuse to admit they might be wrong (or even that they might not know what they think they know), and they just repeat their worn-in misconceptions and crazy ideas over and over like a broken record. That's not skeptical scrutiny and free inquiry, that's the very epitome of closed-mindedness.

So yes. There's something wrong with posting links to obvious crackpot sites: It's explicitly prohibited in the forum rules. And that's an obvious crackpot site. A ridiculously obvious, very stereotypical one, even. There's a reason "Einstein was wrong" has its own http://www.crank.net/einstein.html" [Broken] on crank.net.

Last edited by a moderator: May 4, 2017
11. Sep 20, 2010

### fsoica

I don't even think of hoping tounderstand the relativistic Doppler effect, since relativity is not one of my forte's.
The relativistic approach only "tweaks" the classical one, the one I have trouble grasping.
If light speed is a constant regardless the reference frame, I understand the energy of a photon and so, its frequency, is not. This is, I think, the difficulty for the non-scientist, to understand the photon as a relative entity, to grasp the reality of a light photon, a quanta, being a variable quantity. How come a "green" photon in a reference frame could become red in another one ?

Assuming one understands relativity, it's ok. It's acceptable (I'm guessing here, of course).

But what becomes of the classical effect ? How could I explain this one in layman's terms ?

12. Sep 20, 2010

### fsoica

mate, I understand, I made a mistake, but only with good intentions and only because of my limited knowledge, not allowing me to distinguish crap from real potential. But pls. bear with my curiosity and try to shed some light for me regarding my question...

Last edited by a moderator: May 4, 2017
13. Sep 20, 2010

### fsoica

nice insights...

14. Sep 20, 2010

### Staff: Mentor

If you have taken algebra you should be able to do the process I outlined above. Give it a try and let us know, we will be glad to help if you get stuck.

15. Sep 20, 2010

### rcgldr

It's similar to the dopper effect with sound, which I tried to describe in my previous post. If light source and observer are approaching each other, then the start of each wave cycle from the light source occurs increasingly closer to the observer over time, resulting in an observed shorter wavelength and higher frequency. If the source and observer are moving away from each other, then there is an observed longer wavelength and lower frequency. At relatively low speeds compared to c, the relativistic effect is minimal and the effect is similar to that of sound waves. Link to "classic" wiki articles:

http://en.wikipedia.org/wiki/Blue_shift

http://en.wikipedia.org/wiki/Redshift

16. Feb 5, 2012

### murdakah

Sorry to necro this old thread, but ive been wondering the same thing. I understand the regular doppler effect so dont go on about that. My problem is with regards to light doppler shift.
According to relativity, light's speed is always c in every inertial reference frame, so how is it possible that it imparts less or more energy because of realative motion?
This is particulary difficult when bring particle nature into account.

17. Feb 5, 2012

### Staff: Mentor

In both non-relativistic and relativistic physics, an object's energy and momentum depend on the reference frame (relative velocity) of the observer. If you think of light as a stream of particles with momentum p and energy E = pc, the Doppler effect for light follows from the frame-dependence of energy and momentum (via the Lorentz transformation), together with E = hf.

18. Feb 5, 2012

### murdakah

So in effect more energy is transfered via photons in objects moving closer relative to each other, one being the source and the other the observer. Its logical, but is there a way to imagine it? Since p is the only variable and E is dependent on p. How does p increase in regards to the actual reference frames? Sorry if im reasoning in circles but im having trouble visualising it when c should stay constant.

19. Feb 5, 2012

### Staff: Mentor

If one observer measures an object's momentum and energy as p and E, then another observer who is moving with velocity v with respect to the first observer measures the energy and momentum as

$$p^\prime = \gamma \left( p - \frac{vE}{c^2} \right)$$

$$E^\prime = \gamma (E - vp)$$

where $\gamma = \frac{1}{\sqrt{1 - v^2/c^2}}$

This is the Lorentz transformation for momentum and energy, similar to the L.t. for position and time which you may have seen if you've studied relativity.

For a photon, these two equations are equivalent, as you can see by substituting E = pc and E' = p'c.

20. Feb 5, 2012

### murdakah

Aaah I see. Its what I thought but just didnt have a basis to explain it on. Thanks a lot! :)

21. Feb 5, 2012

### harrylin

Yes that is not allowed, as any link has an effect on such things Google ranking. Thus I also once got a warning (and this explanation) when I referred to a site that the administrators don't want linked.

22. Feb 5, 2012

### harrylin

You are perfectly right: as you say, the relativistic approach only "tweaks" the classical one. The successful classical model on which relativity is based is that of light as a vibration of an invisible medium, and the classical Doppler effect as well as the relativistic corrections rely on that model of "light waves", similar to sound waves. Classically the Doppler equations for sound and light are identical. With that model, perhaps the best way to view a photon is as a kind of wave packet: a wave pulse of limited dimensions. That doesn't change the Doppler effect.

In laymen's terms, just as with the sound of the horn of a train passing by, also the colour of the light that you receive is affected by the velocity of the source as well as by your own velocity. And the tweaks of relativity make that it is always as if there is a light medium that is in rest with respect to the reference frame that you use; only the relative velocity matters for the Doppler calculation.

Last edited: Feb 5, 2012
23. Feb 5, 2012

### James_Harford

If you understand the regular doppler effect, then you can apply that knowledge to the relativistic doppler effect as experienced by any one particular inertial observer. Within that observer's frame of reference, light waves are analogous to sound waves in air provided there is no wind! Anything you can conclude about the change in frequency and wavelength of a moving source of sound has its equivalent for light waves. Einstein's great insight was that any other observers moving at constant speed relative to the first will also find themselves "at rest in the medium".

Edit: I should mention that the analogy applies to frequency and wavelength. The energy of the light photons is the frequency times Plank's constant. Why so, is a question to be answered elsewhere.

24. Feb 5, 2012

### Drakkith

Staff Emeritus
Whoa! I made that post way back when I was but a fresh faced PFer new to the site. Now I am a grizzled old PFer with 5k+ posts under my belt and understand the rules much better. But you are correct in your post, such sites are not allowed to be linked here on PF.