# Does doppler effect affect ENERGY of photon?

1. Feb 23, 2012

### ARAVIND113122

does doppler effect affect ENERGY of photon??

i have tried to make the question as clear as possible,but in the process,have made it a bit too long. i request you to be patient-
suppose a source is emitting a photon after equal intervals of time.an observer,who was initially at rest w.r.t the source,starts to move away from it.this would increase the time interval after which the photon reaches the observer.this means that the frequency of photons reaching the observer decreases,i.e,the apparent wavelength of light increases.

now,energy of a photon is given by e=h√.
suppose the photons emitted by the source have frequency corresponding to that of blue light.when the observer starts to move away,the light,according to the observer,would seem to have shifted towards the red end of the spectrum .

am i right in saying that although the actual frequency of each individual photon[which gives it its energy] remains the same,the frequency PERCEIVED by the human eye decreases?to be more specific,i want to know if the human eye perceives colour based on the INTRINSIC frequency of a photon,or the frequency with which successive photons hit the eye.
if the latter is true,then it wouldn't it mean that an EM wave of ANY frequency can be perceived as that of ANY OTHER frequency by the eye[despite having different energies according to their ACTUAL frequencies{given by E=h√}],depending on the motion of the source w.r.t the source?
or does doppler effect affect the initial frequency of the photon?if so,how?
is the answer a combination of different factors?if yes,how?

2. Feb 23, 2012

### Staff: Mentor

Re: does doppler effect affect ENERGY of photon??

Both the energy and frequency of a photon depend on the observer's reference frame. There is no such thing as "actual" (absolute) frequency or energy, for a photon.

When a photon enters your eye, the sensation of color that it produces depends on its frequency (or energy) in your reference frame.

Indeed, in principle if you accelerate to a high-enough velocity towards an oncoming photon, you can change it (from your point of view) from a green photon to an ultraviolet photon, and even further, to an X-ray or gamma-ray photon.

3. Feb 23, 2012

### Janus

Staff Emeritus
Re: does doppler effect affect ENERGY of photon??

There is no such thing as the INTRINSIC frequency of a photon. The energy of the photon is frame dependent. The energy of a photon and thus its frequency as measured by someone at rest with respect to the source will be different than that it would be for some one moving with respect to the source. Put another way, there is no measurable difference between a "red" photon emit by a source at rest with respect to you and a "red" photon that has undergone Doppler shift.

4. Feb 23, 2012

### ARAVIND113122

Re: does doppler effect affect ENERGY of photon??

okay.consider a case when someone is conducting an experiment to measure current produced by photo-electric effect.the threshold frequency is that of "blue" photons.but the source is emitting "green" photons.then,is it true that if he moves the object at a minimum required speed towards the source,he would observe photoelectric effect??

5. Feb 23, 2012

### Staff: Mentor

Re: does doppler effect affect ENERGY of photon??

Absolutely. The Cosmic Microwave Background itself becomes lethal x-ray/gamma radiation if you accelerate to a high enough velocity.

6. Feb 23, 2012

Staff Emeritus
Re: does doppler effect affect ENERGY of photon??

There's nothing magical about photons. The energy of a photon depends on the observer's velocity. The energy of an automobile depends on the observer's velocity.

7. Feb 24, 2012

### ARAVIND113122

Re: does doppler effect affect ENERGY of photon??

Thank you very much!