Electromagnetic waves

An electromagnetic wave obeys the laws of reflection (equal angles) and refraction at the boundary of two media. A metal surface reflects the wave almost completely: the wave makes the free electrons oscillate and they emit a secondary wave; a dielectric reflects little. In a non-magnetic medium the wave speed is v = c/√ε, so the refractive index is n = c/v = √ε (ε is the permittivity). When the wave passes from one medium to another its frequency ν does not change, while the speed and wavelength do: λ = v/ν. Intensity (radiation flux density) is the energy passing through a unit area perpendicular to the direction of propagation in unit time: I = W/(S·Δt), measured in W/m²; I = w·c (w is the energy density). At distance R from a point source I = P/(4πR²), inversely proportional to the square of the distance; also I ∝ ν⁴, which is why low-frequency currents (e.g. 50 Hz mains) hardly radiate, while high-frequency currents radiate strongly.