Above 100 MHz or so, your wires stop being wires. They become transmission lines, and signals propagate as waves with reflections, standing waves, and an impedance you can no longer ignore. The same physics governs how Wi-Fi reaches your laptop, how a radar paints a plane, how the Hubble streams pictures back from low Earth orbit, and how a TEMPEST attacker reads a CRT through the wall. This is the chapter where Maxwell's equations stop being abstract physics and start dictating the shape of every PCB trace, every connector, and every shield in the modern hardware world.
In Chapter 0 we met Maxwell's equations and saw, in passing, that they predict electromagnetic waves traveling at the speed of light. In Chapter 2 we treated wires as ideal: voltage at one end appears instantly at the other. Both pictures cannot both be true, and in fact neither is exactly right. What reconciles them is frequency-dependent behavior. At low frequencies, wires behave like wires. At high frequencies, every conductor pair becomes a transmission line, and every dielectric becomes a medium for waves. This chapter is about that boundary, and about what to do once you have crossed it.
This is one of the most mathematically dense chapters in the curriculum, but the math is here in the service of physical intuition. Read slowly. Re-read sections. Draw the field pictures. The payoff is enormous: once you internalize traveling waves, characteristic impedance, and reflections, every high-speed digital problem, every RF design, every side-channel attack, and every shielding decision becomes intuitive instead of voodoo.