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Here's my question: If light is an electro-magnetic wave, with oscillating magnetic and electric fields, wouldn't light interfere with the free electrons flowing (slowly) in a wire, as the electric fields of the EM wave meet the electrons? Would this then lead to a change in the current measured in the wire?

The picture in my mind is where light is approaches and touches a live wire, perpendicular to its surface. Here's an old-fashioned PAINT in actionenter image description here.

One of my assumptions is that light can penetrate a live (i.e. with current) wire, in the same way that light penetrates the material in the photoelectric effect from which electrons are emitted.

Here are a two guesses to my own question.

  1. [in terms of EM waves] Since the electric fields of the EM wave are oscillatory - the electric field increases in one direction, decreases, then flips in direction - the journey of the free electrons will only be disturbed a little in one direction and then in the other, to no net effect. Thus the current - as an ammeter would help measure - will be unaffected.
  2. [in terms of physical "intuition", which sometimes is violated in physics!] Since, like how I get warm while sitting under the blazing sun, the wire would be heated by the light, the free electrons will also absorb this heat and so travel with greater kinetic energy. The reading on the ammeter will register an increase in current.

These two guesses reach two mutually exclusive conclusions. Hence my angst and need for your - the reader's - help!

I have a premonition that I have some hidden assumptions that need uprooting ... so do your best, reader!

Note: A10 is a ref number for my own notes. Edit: I added Would this then lead to a change in the current measured in the wire? to my question.

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  • $\begingroup$ Light is EM radiation with very high frequencies 400THz - 960THz, such radiation does not penetrate inside conductors, it is stopped on the surface by dirt (oxides) on the conductor surface and in the very thin skin layer by the conducting electrons (which reflect most of the light and absorb some of it). EM radiation up to GHz range will penetrate a little more and act on part of current that flows in the surface layer of the conductor. Search skin effect. $\endgroup$ Commented Sep 6, 2023 at 1:15
  • $\begingroup$ Thanks @JánLalinský for your comment! Here's a stab to grasp what you just wrote down. The conclusion is that the free electrons in a live wire can be disturbed by light, but the overall current of the wire will not be, since the electrons that absorb the light are only those on the surface of the conductor (and most of the light would just be reflected). In addition, the degree of light penetration into the wire (I use the word "light" to refer all EM waves of all frequencies, not just visible light) will depend on the wavelength/frequency of the incoming light. Am I generalizing too much? $\endgroup$
    – Clueless
    Commented Sep 6, 2023 at 1:57
  • $\begingroup$ Yes. Current will be influenced, but how much depends on the frequency. Radio frequencies - yes, this is how antennas work; optical frequencies - not so much, and there isn't even a way to measure current at such high frequencies. $\endgroup$ Commented Sep 6, 2023 at 8:16

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What you describe here is exactly how radio waves work. Because of the fact that by the acceleration of surface electrons on the antenna rod zillions of polarized photons are emitted, exactly the opposite process happens when hitting a conductor.

The arriving photons - a few out of the huge number of emitted and sufficient already on a much shorter conductor length - move the surface electrons synchronously in one direction because of their polarized state.

Since the acceleration direction of the electrons in the antenna rod is changed periodically - and thus the polarization direction of the photons changes periodically - this also happens at the receiving conductor.

If I build a receiving ferrite antenna, I take advantage of the fact that not only the electric field components of the photons are polarized, but also the magnetic field components.

The fact that light itself cannot cause macroscopic currents in a conductor is due to the fact that even monochromatic light from a thermal source is EM radiation, but has no macroscopic electromagnetic wave properties. And even if the radiation is polarized by means of gratings, there is only a thermal noise at the conductor. The more so that the polarization is incomplete; although the E-field is aligned, both vector directions (plus and minus) are equally distributed.

If I want to induce electric currents with light according to your construction, it must succeed to use synchronously polarized light. If you want to generate current with sunlight, there is of course the traditional way of passing electrons in solar cells through barrier layers with the help of photons.

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    $\begingroup$ Thanks @HolgerFiedler ! I will come back to ask a few questions, but I can see the general sketch. Appreciable changes in a wire's current occur if the incoming EM waves are within a certain regime (by example, radiofrequency regime), otherwise it's insignificant or at best, adds some noise to the current signal. $\endgroup$
    – Clueless
    Commented Sep 8, 2023 at 1:58

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