Chapter 11

# Electromagnetic wave in vacuum

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The electromagnetic wave in a vacuum is studied : ## Question

Is the corresponding wave plane ? Progressive ? Harmonic ? To justify.

What does it remind you of ?

### Solution

It is not plane, but progressive and harmonic.

It is a wave that may be encountered in a waveguide.

## Question

Calculate the magnetic field.

### Solution

We can't use the relationship of structure because the wave is not a plane wave.

We calculate the magnetic field from the Maxwell-Faraday equation : We find : ## Question

Is there any dispersion ?

### Solution

Determining the dispersion relation.

For this, we use the wave equation of the electric field in vacuum (d'Alembert's equation) : We find : There is dispersion with a phase velocity that is : (we assume ) Previous
Electromagnetic waves in vacuum
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Speed of propagation of energy