# Maxwell's Equations: Light as a wave in an electromagnetic field

What does it mean for light to act as a wave in an electromagnetic field? How does an electromagnetic field propagate the information that a change in current has occurred in the form of light waves?

I understand that electrons release photons when they change velocity, and that this may be achieved by alternating the electron flow (changing the current). I would like to understand how these photons act as a wave to propagate the new state of the electron flow to a location far away from the electrons themselves, where the previous state of the electron flow still exists.

Thank you!

This can all be understood without reference to photons.

Consider a transformer, consisting of two parallel loops of wire. A changing current in one loop produces a changing magnetic field threading through the other loop. As the magnetic field changes, it induces an electric field (per the del cross E equation in Maxwell's equations). The electric field induces an electromotive force in the second loop, which drives a current. Similarly, the del cross H equation in Maxwell's equations says that a changing electric field creates a changing magnetic field. Changing E induces changing H which induces changing E, and so on---; and that is what an electromagnetic wave is all about.

• Thank you for responding! I think the part I am having trouble wrapping my head around is how does the induced magnetic field not change the original (generated) electric field that produced it (which would, in turn, cause the change in the induced magnetic field to be different in the first place, making it impossible to have changed the original (generated) electric field). Is there some sort of delay by which an electric field induces a magnetic field? – patrikperko Feb 26 '18 at 21:18

If your familiar with Maxwell's equations then you must know:

A changing magnetic field produces an electric field
A changing electric field produces a magnetic field

Following up, when a current in a wire changes the corresponding magnetic field also changes. This change in magnetic field causes an electric field to be produced.The electric field produced is such that it also changes. So another changing magnetic field is produced. Which again produces an electric field.

While this happens with each jump from magnetic field to electric field the wave moves forward. Hence it can travel a far distance before it is absorbed.

• Thank you for your answer! What do you mean by "The electric field produced is such that it also changes"? Wouldn't this produced electric field also change the original magnetic field that changed it? I don't understand how to establish a continuity here. Is there some delay in how an electric field responds (by changing) to a changing magnetic field? – patrikperko Feb 26 '18 at 21:09
• @patrikperko I quite didn't get what you said but the magnetic field produced is a time varying sine wave which produces another time varying electric field and the subsequent loops have a phase difference in them. (but the magnetic and electric field have same phase in the same loop) – SmarthBansal Feb 27 '18 at 5:08
• From what I understand, an electric field is simply "information about how charge is arranged." Therefore, when this charge arrangement changes, the "information about how charge is arranged" (a.k.a. the electric field) changes as well. How is this change in the "information about how charge is arranged" propagated to a far away location? What "carries" the "change in electric field" information to this far away place? Comparatively, the medium of a water wave (water molecules) "carries" the information of the "water wave" through the cause-effect movement of the individual water molecules. – patrikperko Feb 27 '18 at 7:20