Let’s say a green laser is shone onto a target ten meters away. Suppose we look at a photon as it attempts to travel to the target. Please correct my assumptions as needed. Assumptions: 1. It could get “lucky” and miss all of the nitrogen and oxygen atoms. In this case it would be essentially traveling in a vacuum and travel at the full speed of light. 2. It could hit an atom indirectly causing the photon to scatter or disperse and not reach the target. 3. It could hit either a nitrogen or oxygen atom directly transferring all of its energy to the atom, but due to the atom’s structure and the photon’s energy level a new photon is not emitted. I believe it takes at least a Ultraviolet photon to get a photon out of a nitrogen or oxygen atom.

The problem is there no event that simply slows the light to the refractive index air. What am I missing? It seems like all of the events either misdirect, stop or have no affect on the photon.

  • $\begingroup$ Curiosity more than knowledge: dust? $\endgroup$
    – user167453
    Oct 7, 2017 at 18:06
  • $\begingroup$ en.wikipedia.org/wiki/Permittivity is my second guess, but I am only tbis studying now, as a possible contribution to the effect, so I would like to see if this is correct. $\endgroup$
    – user167453
    Oct 7, 2017 at 18:17

1 Answer 1


So I am not a quantum field theorist. But what one would likely tell you is that at the single photon level, you need to think about the transmission "quantum mechanically". That is, the photon has a wavefunction which travels all the possible paths from emitter to detector simultaneously. Add the complex amplitudes of each of these possibilities to determine what the photon actually does. In this way, it interacts with all the atoms simultaneously. The coherent interference of all these interactions/paths gives you an effective ("dressed") photon which has all the properties of the original photon, except a somewhat shorter wavelength, larger momentum, and of course slower speed (more precisely, phase velocity) through the medium. You can think of it like a mess of coherent absorptions and re-emissions that follow the "photon" and slow it down as it propagates to the detector.

In any case, be careful about how you're thinking of the "photon's" wave-like and particle-like properties. Because, if you simply imagine a photon like a little ball flying through space and maybe bumping into things, you'll run into conceptual difficulties. After all, what is a photon other than the transmission of a packet of energy and momentum from point A to point B, as dictated by the interference of a wavefunction?

At least, that's how the QED formalism deals with it (I think). What is actually happening is a matter of interpretation. I personally like the transactional interpretation of quantum mechanics, and I'll leave it as an exercise for the reader to determine how TI would explain this photon transmission process.

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    $\begingroup$ Thanks Gilbert. Ironically I find myself using the same strategies to understand QM that I use to understand religion: trust in authority, overlook obvious challenges to common sense and give things supernatural qualities, such as being in two places at the same time. It’s odd to me that it is now science that requires faith. $\endgroup$
    – Lambda
    Oct 8, 2017 at 5:39
  • $\begingroup$ I wonder if the dressed photon’s shorter wavelength would be visible as changing from green to a greenish blue. $\endgroup$
    – Lambda
    Oct 8, 2017 at 13:48
  • $\begingroup$ @Lambda I don't think there's anything wrong with faith in scientific authority to some extent, because it is really a faith in the scientific process and the motivations of its practitioners to ultimately lead to an experimentally verifiable framework. In principle, one doesn't need blind faith in the "spookiness" of QM, since the mathematical framework has been experimentally confirmed many times, and you could simply repeat one of those measurements if you wanted. Interpretations, though, do require blind faith, in so much as they are experimentally indistinguishable. $\endgroup$
    – Gilbert
    Oct 8, 2017 at 15:08
  • $\begingroup$ @Lambda ahh, and regarding your second question, the color you perceive is actually related to the frequency of the light rather than the wavelength. Check out my answer to this other question here: physics.stackexchange.com/questions/352217/… $\endgroup$
    – Gilbert
    Oct 8, 2017 at 15:34
  • $\begingroup$ Good points. I’ve always wonder if it was frequency or length good to finally know. Thanks $\endgroup$
    – Lambda
    Oct 8, 2017 at 15:39

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