Displacement current Id = ε_{0}(dΦ_E/dt) is introduced by Maxwell to fix Ampere's law for charging capacitors. It is NOT actual charge flow — it arises from a changing electric field. It equals the conduction current in the external wire and produces a magnetic field just like a real current.

Modified Ampere-Maxwell law: ∮B·dl = μ_{0}(I_c + ε_{0} dΦ_E/dt). This ensures current continuity throughout a circuit containing a capacitor.

Maxwell's four equations describe all of classical electromagnetism: (1) Gauss-E: charges produce E fields; (2) Gauss-B: no monopoles, B lines closed; (3) Faraday: changing B → E; (4) Ampere-Maxwell: currents and changing E → B.

EM wave origin: Accelerating charges produce EM waves. The mutual coupling of equations (3) and (4) — changing B creates E, and changing E creates B — enables self-sustaining propagation.

EM wave in vacuum:

• Transverse: E ⊥ B ⊥ propagation direction
• In phase: E and B reach peaks and zeros simultaneously
• Universal speed: c = $3 \times 10^{8}$ m/s for ALL frequencies (no exceptions in vacuum)
• Amplitude ratio: $E_{0}$/$B_{0}$ = c
• Formula for c: c = 1/√(μ_{0}ε_{0})

Energy and momentum: Intensity I = (1/2)ε_{0}c$E_{0}^{2}$ [W/$m^{2}$]. Momentum for absorption p = U/c; for total reflection p = 2U/c. Radiation pressure exists for all EM waves.

EM spectrum order (increasing frequency): Radio → Microwave → Infrared → Visible → UV → X-rays → Gamma. Mnemonic: "Rich Men In Visible Uniforms eXude Glamour."

Visible spectrum (VIBGYOR): Violet (400 nm, highest f) to Red (700 nm, lowest f).

X-ray vs Gamma: Frequency ranges overlap (10^{18}–10^{21} Hz). Distinguished by source only: X-rays from electron deceleration/atomic transitions; gamma rays from nuclear decay.

In medium: EM waves experience dispersion — different wavelengths travel at different speeds (v = c/n). This does NOT happen in vacuum.