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I'm reading about optical waveguide analysis, and often come across the terms "transverse electric mode" vs. "transverse magnetic mode". As I unerstand, it means that the electric/magnetic field has only transversal component.

But how could it have longitudinal component? Aren't the electric and magnetic fields always perpendicular to the direction of propagation?

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  • $\begingroup$ "Aren't the electric and magnetic fields always perpendicular to the direction of propagation?" In free space, yes. However, waveguides can support waves with longitudinal field components. I seem to recall that most texts do an example of one of those, but it has been a long time... $\endgroup$ May 1, 2013 at 15:49
  • $\begingroup$ Electrostatic waves have $\mathbf{k} \ \times \ \mathbf{E}$ = 0, but then again, I think they can only exist in a medium, not a vacuum. Hmm... $\endgroup$ Oct 3, 2014 at 18:07
  • $\begingroup$ @dmckee not even in free space. If you form a finite beam it will have a small longitudinal component. It's true that plane waves in free space have no longitudinal component but plane waves do not exist in nature. $\endgroup$
    – JohnS
    Apr 17, 2018 at 14:27

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