Class 12 Physics Chapter 8 Electromagnetic Waves – Revision Notes

Electromagnetic waves emerge once Maxwell adds displacement current to Ampere’s law, and this Class 12 Physics Chapter 8 summary traces that idea through the properties of the waves and the electromagnetic spectrum.

Displacement Current

• Maxwell added a term, I_d=ε₀(dΦ_E/dt), to Ampere’s law so that a changing electric flux acts as a source of magnetic field, just like conduction current.
• Total current (conduction + displacement) is always continuous, even where conduction current alone seems to stop (e.g. between capacitor plates).
• Displacement current has the same SI unit as conduction current: the ampere (A).

Properties of Electromagnetic Waves

• E and B oscillate sinusoidally, perpendicular to each other and to the direction of wave propagation, and in phase with each other.
• They travel through vacuum at the speed of light, c=1/√(μ₀ε₀)≈3×10⁸m/s, and require no medium.
• Amplitude relation: E₀=cB₀.
• Average energy density: u_E=¼ε₀E₀² (electric part), u_B=¼B₀²/μ₀ (magnetic part); u_E=u_B, so each field carries exactly half the wave’s total average energy.
• Electromagnetic waves carry momentum and can exert radiation pressure.

The Electromagnetic Spectrum

• Ordered by increasing wavelength (decreasing frequency and photon energy): gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, radio waves.
• Photon energy E=hν spans roughly 15 orders of magnitude across the spectrum, from a fraction of a µeV for radio waves to millions of eV for gamma rays — reflecting the very different physical processes (oscillating currents, molecular vibration, atomic transitions, nuclear transitions) that generate each band.
• Visible light is only a narrow slice of the full spectrum, roughly 4000–7000Å (400–700nm) in wavelength.

One-Line Summary

A changing electric field produces a magnetic field and vice versa, and this mutual regeneration lets electromagnetic waves — from radio waves to gamma rays — propagate through vacuum at the speed of light, carrying energy split equally between their electric and magnetic parts.