EM Wave Propagation

An electromagnetic wave is a self-propagating, transverse oscillation of coupled electric (E) and magnetic (B) fields.

E ⊥ B ⊥ direction of propagation. E × B always points along the propagation direction (Poynting vector).

All EM waves travel at the SAME speed in vacuum: c = 1/√(μ₀ε₀) ≈ 3 × 10⁸ m/s — independent of wavelength, frequency, or amplitude.

E and B are in phase: when E peaks, B peaks at the same point and time.

Ratio of field amplitudes: E₀/B₀ = c. So B is much smaller numerically than E (in SI units), but they carry equal energy density.

In a medium of refractive index n: v = c/n. Frequency f stays fixed (set by the source); wavelength shrinks to λ_medium = λ_vac/n.

EM waves DO NOT need a medium — Maxwell's prediction was confirmed by Hertz's 1887 experiment using spark-gap oscillators.

EM waves carry both energy (Poynting vector) and momentum (radiation pressure).