Electromagnetic Waves Revision Notes for NEET Physics PDF Download
📡 ELECTROMAGNETIC WAVES · EM Waves
Maxwell's equations • Displacement current • Properties • EM spectrum • NEET problems
⚡ MAXWELL'S EQUATIONS & DISPLACEMENT CURRENT
Displacement current: Id = ε₀ dΦE/dt
Ampere‑Maxwell law: ∮ B·dl = μ₀(Ic + Id)
Maxwell's equations unified electricity, magnetism, and light.
📐 Displacement current in a capacitor
🌊 ELECTROMAGNETIC WAVES
EM waves are transverse waves consisting of oscillating electric and magnetic fields perpendicular to each other and to the direction of propagation.
Speed in vacuum: c = 1/√(μ₀ε₀) ≈ 3×10⁸ m/s
Relation: E0 = c B0 (amplitudes are related)
Energy density: u = ½ ε₀ E² + ½ B²/μ₀ = ε₀ E² (since B = E/c)
🖍️ EM wave propagation
🌈 ELECTROMAGNETIC SPECTRUM
Order of increasing frequency (decreasing wavelength):
Radio waves → Microwaves → Infrared → Visible → Ultraviolet → X‑rays → Gamma rays
Radio waves
λ > 0.1 m, used in communication, TV, radio.
Microwaves
λ ~ 1 mm – 0.1 m, radar, ovens, satellite communication.
Infrared (IR)
λ ~ 700 nm – 1 mm, heat, remote controls.
Visible light
λ ~ 400 – 700 nm (VIBGYOR).
Ultraviolet (UV)
λ ~ 10 – 400 nm, causes tanning, germicidal.
X‑rays
λ ~ 0.01 – 10 nm, medical imaging.
Gamma rays
λ < 0.01 nm, nuclear radiation, cancer therapy.
📊 Hand‑drawn EM spectrum
💥 ENERGY & MOMENTUM
Energy density: u = ε₀ E² = B²/μ₀
Intensity (power/area): I = u c = ε₀ E² c = (E₀²)/(2μ₀ c) (for sinusoidal)
Poynting vector: S = (1/μ₀) E × B (direction of propagation)
Radiation pressure: Prad = I/c (if fully absorbed), = 2I/c (if reflected).
Momentum: p = U/c (for photon)
🔧 PRODUCTION & DETECTION
EM waves are produced by accelerating charges (e.g., oscillating dipole, antenna).
Detection: antenna, photoelectric effect, etc.
💡 NEET TIPS & SHORTCUTS
- For EM wave, E and B are in phase; the wave is transverse.
- Speed in a medium: v = c/n, where n = √(μrεr) ≈ √(εr) for non‑magnetic.
- Displacement current is essential for continuity of current in capacitor circuits.
- Order of EM spectrum: remember "Rabbits Mate In Very Unusual eXpensive Gardens" (Radio, Micro, IR, Visible, UV, X‑rays, Gamma).
⚠️ COMMON MISTAKES
- Confusing direction of E and B fields in propagation.
- Forgetting that displacement current exists even in vacuum (no actual charge flow).
- Using c = 3×10⁸ m/s in medium without considering refractive index.
- Assuming EM waves require medium – they don't (can travel in vacuum).
📌 QUICK REVISION CARD
c = 1/√(μ₀ε₀) ≈ 3×10⁸ m/s
E₀ = c B₀
Displacement current: Id = ε₀ dΦE/dt
Intensity: I = ½ ε₀ c E₀²
Poynting vector: S = (E × B)/μ₀
Radiation pressure: P = I/c (absorbed), 2I/c (reflected)