# What exactly is the definition of an EM wave?

Do we say something is an EM wave only if the EM field is oscillating at a constant frequency? What exactly is the definition of an EM wave?

If an electron moves in a direction and then stops moving, is an EM wave produced by that electron?

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ShayanJ
Gold Member
An EM wave is a change in the EM field propagating in space. Its a signal that tells different parts of the space to update their value of the field. Any acceleration present in the charge distribution causes an EM wave. Changing magnetic fields may cause EM waves too.

Nugatory
Mentor
Do we say something is an EM wave only if the EM field is oscillating at a constant frequency?
No. The wave equation is linear, so any two of its solutions can be added to yield a third. Electrical and magnetic fields also add, so if I have two oscillators at different frequencies, the resulting electrical and magnetic field will also be a solution to the wave equation. It won't be a neat pure sine wave with a definite frequency, and by adding enough different frequencies and amplitudes you can produce almost arbitrarily complicated waveforms.

What exactly is the definition of an EM wave?
I don't know that there's a single exact definition, but I'd expect just about everyone to agree that any time-varying electromagnetic field that can be written as a sum of terms containing ##e^{i(kx-vt)}## would be one.

If an electron moves in a direction and then stops moving, is an EM wave produced by that electron?
Yes. It will be a single pulse, but it will be a wave.

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sophiecentaur