Maxwell introduced displacement current Id = ε_{0}(dΦ_E/dt) to resolve an inconsistency in Ampere's law for a charging capacitor, where no charges cross the gap but the electric field changes.

Displacement current is not actual charge flow — it is produced by a time-varying electric field and equals the conduction current in the external wire, ensuring current continuity.

Maxwell's four equations — two Gauss laws, Faraday's law, and the Ampere-Maxwell law — form a complete and symmetric description of electromagnetism.

The symmetry between Faraday (changing B → E) and Ampere-Maxwell (changing E → B) enables self-sustaining electromagnetic wave propagation.

EM waves are transverse: the electric field, magnetic field, and direction of propagation are mutually perpendicular, and E and B oscillate in phase.

All EM waves travel at the same speed c = 1/√(μ_{0}ε_{0}) = $3 \times 10^{8}$ m/s in vacuum, independent of frequency — a universal constant.

The amplitude ratio $E_{0}$/$B_{0}$ = c, intensity I = (1/2)ε_{0}c$E_{0}^{2}$, and radiation momentum p = U/c (absorption) or 2U/c (reflection) are the three key EM wave relationships.

The EM spectrum spans from radio waves (lowest frequency) through microwaves, infrared, visible, ultraviolet, X-rays, to gamma rays (highest frequency).

X-rays and gamma rays have overlapping frequency ranges and are distinguished only by source: X-rays from electron deceleration or atomic transitions; gamma rays from nuclear decay.

Application pairings critical for NEET: microwaves → RADAR/ovens; UV → sterilisation/LASIK; IR → night vision/physiotherapy; X-rays → medical imaging; gamma → cancer treatment.