Chapter 8 — Electromagnetic Waves

Electromagnetic waves are coupled oscillating electric and magnetic fields that propagate through space at the speed of light (3×10⁸ m/s) in vacuum, arising from Maxwell's equations and produced by accelerated charges.

Class 12 Physics Chapter 8 covers electromagnetic waves, beginning with Maxwell's concept of displacement current — an additional current (id = ε₀ dΦE/dt) introduced to resolve an inconsistency in Ampere's circuital law. Maxwell's equations predict that accelerated charges radiate electromagnetic waves in which E and B fields oscillate perpendicular to each other and to the direction of propagation, travelling at c = 1/√(μ₀ε₀) ≈ 3×10⁸ m/s. The chapter also covers the full electromagnetic spectrum from gamma rays (wavelength ~10⁻¹² m) to radio waves (~10⁶ m), including their sources, properties, and practical applications.

1. 01Maxwell introduced displacement current (id = ε₀ dΦE/dt) to fix an inconsistency in Ampere's circuital law when applied to a charging capacitor.
2. 02Accelerated (not stationary or uniformly moving) charges produce electromagnetic waves; the wave frequency equals the frequency of charge oscillation.
3. 03In an electromagnetic wave, E and B are perpendicular to each other and to the direction of propagation, related by E₀/B₀ = c.
4. 04The speed of EM waves in vacuum is c = 1/√(μ₀ε₀) ≈ 3×10⁸ m/s; in a medium it becomes v = 1/√(με).
5. 05Hertz first experimentally demonstrated electromagnetic waves in 1887; Jagdish Chandra Bose later produced shorter-wavelength EM waves (25 mm to 5 mm).
6. 06The electromagnetic spectrum spans gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves — all travelling at c in vacuum but differing in wavelength, source, and interaction with matter.