# What can change a photon's frequency?

I found this question: Is it possible to apply force to a light particle?

As it states, gravity can change the frequency of light by changing its momentum. My question regards other phenomena that could change the frequency of light, and, as commented on the answer there, there are two cases:

1. The photon remains intact - that means only its frequency changes.

2. A process that consumes one photon and produces another at a different frequency.

What I ask myself is whether the propagation of light from one medium to another could somehow change its frequency and whether we can find other phenomena in general (one is gravity) that might change the frequency of a photon.

Some suggestions from the answer of the mentioned question are that the frequency can be changed by the expansion of space, spontaneous pair-production, scattering and red/blue-shifting events. So if you can elaborate on these suggestions regarding the two cases mentioned above, you would help me a lot.

That is, is possible to change light's frequency regarding the two cases above and are there phenomena like those mentioned that could produce such a change?

Note: I have also found this question, but I ask for a more general treatment than only light propagating in air and I'm concerned with phenomena that might have such effects.

• Can you be more specific about what is your question? There's only one question mark in your post, and it is in the title of the older post that you linked to. May 25, 2015 at 16:23
• @ThePhoton You are correct, I got distracted. I've made an addition. Is the question better phrased now? Thanks. May 25, 2015 at 16:27
• How strictly are you demanding that the photon remain intact? Does the process need to preserve the exact quantum state of the system? Or do you only care about the classical properties? If the latter, stimulated parametric down conversion can sort of do it (but it turns superpositions into entangled states). If the former, you can couple into an exciton-polariton cavity and allow it to thermalize, but the redshift is tiny. May 25, 2015 at 16:53
• When light changes frequency, whether or not you think of it as changing the frequency of a photon or removing the first one and creating another one at the new frequency is mostly up to you. It might make sense to think of it as a single photon if the process is adiabatic. May 25, 2015 at 17:37
• The process of quantum frequency conversion can, at least in principle, simply change the frequency of a single photon while preserving its quantum properties. May 25, 2015 at 18:04

The photon is an elementary particle.

There are two ways to measure the frequency and therefore the energy of the photon since its energy E=h*nu .

1. using a diffraction grating which analyses the wavelengths in a beam of light , as below:

This is the spectrum of iron.

Each line is composed of zillions of photons with that frequency. If one sent one photon at a time to the grating , it would slot in the correct location for its wavelength and therefore its frequency would be known.

2)by knowing its interactions with other elementary particles, then from energy conservation its energy will be known and so its frequency too. This happens in particle physics, where the photon can interact with other elementary particles and scatter as in the blue sky, or through Compton scattering, losing part of its energy and momentum on an electron or nucleus and change its frequency. It can also disappear completely in creating pairs of other elementary particles.

Diffraction gratings have shown changes in the spectra coming from stars and galaxies, of known atoms compared to the ones in earth, with blue shifts and red shifts.

The analysis of these spectra shows that the change in frequency is due either to the velocity of the star/galaxy with respect to us, or to the effect of a gravitational well. The latter has been measured in on an earth experiment. When these are taken into account, an overall red shift that shows that everything is receding from the earth has led to Hubble's law and the need for an expanding universe.

• Hello. Thanks for the answer. May I ask this: Isn't scattering in the atmosphere a change in wavelenght(at least in classical electrodynamics) and if so there a disconnection between CED and QM or am I mistaken somehow? Thanks again. May 26, 2015 at 8:23
• If you read the link provided you will see that the blue becomes dominant because it is elastically scattered with higher crossection " This scattering, called Rayleigh scattering, is more effective at short wavelengths (the blue end of the visible spectrum). Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun's light is predominantly in the blue end of the spectrum. " there is no discrepancy as QM takes care of individual scatters ( the more and the less in the phrase) May 26, 2015 at 9:34
• As frequency does not change in Rayleigh scattering, there seems to be no exchange in momentum (Newton's actio/reactio). I wonder what the changes are for the scattering particle; that should be some change of direction only. If the scattered particle only changes direction (no change of c, velocity), can this translate into any change of velocity or even mass/energy on the side of the scattering particle? Would that make up some other question? Nov 16 at 7:35
• @PeterBernhard note (wiki first paragraph) "elastic scattering of light or other electromagnetic radiation", "light" is classical electromagnetic wave. If down into particles and fields the question enters quantum mechanics and many different contribution of Feynman diagrams can enter, and concepts of center of mass of photon and oarticle/field. it is not simple Nov 16 at 7:47
• Thank you: if it's understood that "direction" of the photon can be transfered into "other" energy of the defracting particle or mass-centre just let me know. Likewise if the change of angle does not amount to energy. By the way: The first link you give seems broken. Nov 16 at 11:14

In regards to your question: Can the frequency of light change in propagation from one media to another, the answer is no. I found a previous response to a similar question that might help you:

Think of it like this: At the boundary/interface of the medium, the number of waves you send is the number of waves you receive, at the other side, almost instantly. Frequency doesn't change because it depends on travelling of waves across the interface.

But speed and wavelength change as the material on the other side may be different, so now it might have a longer/shorter size of wave and so the number of waves per unit time changes.

• Thanks. But, is there a mathematical proof for this considering reflection and transition(of course we are talking classical electrodynamics here)? May 25, 2015 at 17:17
• I think it follows from the continuity of the electric field (assuming no charge on the surface) and maybe some Fourier arguments May 25, 2015 at 18:25
• There is some contradiction in the quote to support this: "... frequency does not change" (because of light travelling), then "speed and frequency changes" because the material is different. I wish there were some reference to some textbook in that quote. Did you find any other resources? Nov 16 at 7:44

Gravity can change frequency. A light beam going towards a massive body is blue-shifted by the gravitational field. If it is escaping a gravitational body, then it is red-shifted.

• Hi. Thank for the reply. Is this a phenomenon belonging to classical field theory -relativity or in quantum mechanics or finally both(and if so are there differences)? May 26, 2015 at 8:25

Here we need to be careful with the fundamental idea of energy conservation while thinking that photons remain intact and frequency changes or that a process consumes a photon and produces another of a different frequency. The frequency of a simple pendulum depends only on the length of the string and the change in gravity. In the photon case, it was told frequency also depends on the gravitation field!