# It is said that electromagnetic waves travel only through electric or magnetic fields whose directions are perpendicular. What does this mean?

http://www.astronomynotes.com/light/emanim.gif

Electric and magnetic fields oscillate together but perpendicular to each other and the electromagnetic wave moves in a direction perpendicular to both of the fields.

Electromagnetic waves consist of electricity and magnetism. The energy in an electromagnetic wave is tied up in the electric and magnetic fields. The last two of the free space Maxwell's equations imply that the wave of the electric field is in phase with and perpendicular to the magnetic field wave.

That picture gif may be in error. The "changing" dv/dt, E field creates changing M field and vice versa. When E is max, M is min, and vise versa.

Actually, the picture is correct. dE/dt does not give the magnetic field directly, it gives the curl of the field: dE/dt = mu_0*epsilon_0*dB/dz) for a wave propagating in the z direction with E in the y direction and B in the x direction. Therefore B is maximum at the same place and time where E is maximum. In other words, E and B are in phase.

Another way to see this is that the Poynting vector, S = EXB /mu_0, gives the power density transported by the wave. If E and B were 90 degrees out of phase, then S would average to zero, and light would not transport energy, contradicting the experience of anyone who has stepped barefoot on black asphalt pavement on a sunny day. In contrast, electromagnetic STANDING waves do have E and B 90 degrees out of phase, since a standing wave is not transporting energy anywhere.

Hope this helps!

--Stuart Anderson