Wave impedance of free space

Wave impedance is the fixed ratio between the strength of the electric field and the strength of the magnetic field in a travelling electromagnetic wave — a kind of 'exchange rate' that vacuum itself sets. Just as Ohm's law in a wire ties voltage to current through a resistance, this number ties the E-field to the H-field through a quantity that behaves exactly like a resistance, even though no wire is involved. For a wave cruising through empty space the value is a tidy 377 ohms, a constant baked into the fabric of the universe.

This 377 Ω is why antenna and microwave engineers obsess over matching: when a wave crosses from one medium to another whose wave impedance differs, part of it bounces back instead of passing through, exactly like light reflecting off a pane of glass. Match the impedances and the wave glides across with no reflection; mismatch them and you waste power and create standing waves. The number drops inside materials — in a dielectric with permittivity ε it becomes 377/√εr — which is the seed idea behind radar-absorbing 'stealth' coatings and anti-reflection layers.