Comparing EM Wave Behaviour: Vacuum vs. Medium
| Property | In Vacuum | In a Medium |
|---|---|---|
| Speed | c = m/s (same for ALL frequencies) | v = c/n < c (n = refractive index) |
| Frequency dependence of speed | None — all travel at c | Yes — dispersion occurs (different λ → different v) |
| / ratio | = c | = v (phase velocity in medium) |
| E–B phase relationship | In phase (peaks simultaneous) | In phase |
| Nature | Transverse | Transverse |
| Polarisation | Possible | Possible |
Comparing Absorption vs. Reflection (Radiation Pressure):
| Scenario | Momentum Transferred | Radiation Pressure |
|---|---|---|
| Complete absorption | p = U/c | P_rad = I/c |
| Total reflection | p = 2U/c | P_rad = 2I/c |
Comparing EM vs. Mechanical Waves:
| Feature | EM Waves | Sound (Mechanical) |
|---|---|---|
| Need medium? | No (propagate in vacuum) | Yes |
| Nature | Transverse | Longitudinal |
| Speed in vacuum | c = m/s | Does not propagate |
| Can be polarised? | Yes | No |