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I’m a 12th grade student and I’ve had this question for at least a year. This is lenz’s law as I know it: Change in the magnetic flux associated with a coil induces a current, which opposes the cause that is producing it.

My question is, why can’t light cause this change in magnetic flux? It has an oscillating electric and magnetic field. I’ve heard that the the oscillating electric field can oscillate electrons (it’s used in TVs or smth), so why can’t the oscillating magnetic field induce current?

I had a crazy idea that light does actually induce an EMF which opposes the cause (which is light), and this is the reason light slows down in a medium. That is, of course, till reality struck, there would be electric currents everywhere the sun is up. I decided to wait till I get to know light better but I’m really not getting close to an answer.

Shreya
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  • Most sources of light are incoherent and the averaged EM field at a given point due to these propagating light rays is practically zero. So you shouldn't expect electric currents everywhere – KP99 Nov 02 '21 at 13:38
  • @KP99 so if we use a coherent source of light, will it induce current?? And do you think this current could cause light to slow down (because the oscillation would be opposed and thus slowed down)? – Shreya Nov 02 '21 at 13:48
  • Yes, in presence of charged particles it will induce some non-zero EMF. This propagating light will produce some back-reaction and this leads to phenomena such as refraction, so your idea is partially correct but incomplete. See this explanation by Dr. Don : https://www.youtube.com/watch?v=NLmpNM0sgYk – KP99 Nov 02 '21 at 14:04
  • You are right. See How thin can the wires of a sub-wavelength gold mesh be and still be half as reflective as a solid sheet? What physics dictates this? – mmesser314 Nov 02 '21 at 14:22
  • @KP99 and mmesser314 thank you! The links you shared use the electric field of light for inducing current though, is my explanation (using the magnetic field) correct? I’ve seen videos like the one KP99 suggested and I rewatched a few, they’re actually how I had thought of the question initially, but they explain the slowing of light this way: “the electric field of light oscillates electrons which emits light that interferes with the original light, slowing it down.” I want to know whether my explanation (using magnetic field and Lenz’s law) is correct too – Shreya Nov 02 '21 at 14:42
  • Lenz's law is already incorporated in Maxwell's equations (which appears as the minus sign in RHS of equation $\nabla \times E = -\frac{\partial B}{\partial t}$). Explanation by Dr. Don also applies to magnetic field since electric and magnetic fields are mutually perpendicular inside matter (but may be out of phase). So your explanation is correct but incomplete. Light is electro-magnetic wave and you will need to take into account contributions from both electric and magnetic fields – KP99 Nov 02 '21 at 15:06

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In simple terms, light is comprised of an oscillating magnetic field which induces an oscillating electric field which induces an oscillating magnetic field. That's how it propagates. This is described by Maxwell's equations in vacuum.

A form of electromagnetic radiation which induces electric currents is when radio waves strike an antenna. The currents then travel in the antenna to a receiver.

However, electric field and electric potential are different from electric currents. You can have an electric field with negligible electric currents. Electric fields cause electric currents depending on resistivity or conductivity. The common equations describing this relationship are:

$J=\sigma E$ or $E = \rho J$

In a perfect vacuum (and let's ignore high energy things like pair production), there are literally no charge carriers so no electric currents. But there is an electric field.

Alwin
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  • Thanks for the quick reply! I understand that electric field and electric current are different, how does it answer the question though? Could you elaborate further – Shreya Nov 02 '21 at 13:45
  • @Shreya The answer is a little buried here. Light does induce currents--that's what the antenna example illustrates, because radio waves are electromagnetic radiation as much as visible light. One can also model surface currents induced by visible light hitting conductive surfaces as the cause of reflectivity (why metals are shiny). – Rick Goldstein Nov 02 '21 at 14:03
  • @RickGoldstein could that induced current be due to the magnetic field though (not the electric field), and then can we ascribe the slowing of light in a medium to Lenz’s law? The explanations I found online use the electric field of light to explain its slowing, but is my explanation correct too? – Shreya Nov 02 '21 at 14:34
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    You ask if light induces an emf because of the oscillating magnetic field. I answer that yes, light is comprised of an induced emf because of the oscillating magnetic field, and that this induced emf causes a propagating oscillating magnetic field in turn. The two are coupled. In light, you cannot have one without the other. That is why light is referred to as "electro-magnetic radiation." – Alwin Nov 02 '21 at 15:50