Do E and B Fields Distort When Focusing Photons with a Lens?

[jmatejka]

I have read different light frequncies have different lensing characteristics. Example, Gamma frequencies not capable of being focused with a lens.

This made me think, what happens to E and B fields, for visible spectrum, when you focus photons with a lens? Anything? Do the fields distort, or are the photons doing their "own thing"? I don't recall this ever being specifically addressed in any of my undergrad Physics courses.

Any insight is appreciated, Thanks!

 

[Claude Bile]

Gamma frequencies can't be focused by ordinary lenses because of the dispersion of ordinary glasses (i.e. how the refractive index varies with wavelength). This issue is a bit different to that discussed below.

Classical physics describes perfectly well how E and B fields "distort" in the presence of a lens, we don't need to invoke the concept of photons to explain this behaviour. Look up Gaussian optics for further insights.

Claude.

 

[Ken G]

I have read different light frequncies have different lensing characteristics. Example, Gamma frequencies not capable of being focused with a lens.

Not a lens made of glass, but you could focus gamma rays with a gravitational lens. So it's more of a detail in how the focusing is happening, more so than a rule about gamma rays.

This made me think, what happens to E and B fields, for visible spectrum, when you focus photons with a lens?

This is mixing two different languages for talking about light, the classical picture of E and B fields, and the quantum mechanical photons. The classical treatment normally suffices to understand what large numbers of photons will do, or what individual photons are most likely to do, so normally focusing is an effect that is calculated with the fields, and the fields imply a propagation direction, and the photons follow that. If one wanted to do the calculation with photons from the start, it would be much harder, but you would look at how the photon wave functions are affected by the presence of the medium, and you would find that their "phase velocity" gets slowed by the medium. Then you would ask what this does to the constructive interference between all the different paths the photon could take, and you find it bends the path of constructive interference, causing a focusing effect.

So, if you were considering photons, you'd never ask about the E and B fields (you'd just engineer them in for many photons after you knew what each photon was doing), and if you were considering E and B fields, you'd never ask about the photons (you'd just engineer them in after you knew what the macroscopic fields were doing). This is typical in physics-- more so than having a description of what is "actually going on", we instead select a given approach to treating what is going on, and these approaches are informed by their success in practice, more so than by virtue of being a complete description of reality.

 

[jmatejka]

Starting to make some sense, Thanks!

 

[jmatejka]

Just to confirm. At (single) photon emission, E and B fields accompany.

If this (single) photon is then absorbed, this photon's particular E and B fields "disappear" at time of absorbtion?

If the photon stops propegating, so does it's fields, correct?

If you were to collapse the E or B field, the photon disappears?

 

[jtbell]

If you have a situation where there is only a single photon (or a few of them), it's not meaningful to talk about the classical E and B fields. The E and B fields of an electromagnetic wave "emerge" as the classical limit of the collective effect of bazillions of photons.

If you have a very large number of photons, then there are ways to make a correspondence between the collection of photons and the classical E and B fields. For example, if you know the energy (joules) of electromagnetic radiation in a certain volume, you can calculate the number of photons in that volume, or the average electric and magnetic field strengths of the electromagnetic waves.

But this does not mean that you can take little "pieces" of those E and B fields and attribute them to individual photons.

 

[jmatejka]

Thanks, much appreciated!