Class 11 Physics — Chapter 307: Electromagnetic Waves and Communication System

Key concepts: Electromagnetic Waves and Communication System (Class 11 Physics)

Maxwell's completion of electromagnetism and its consequence: displacement current and the corrected Ampere law, the prediction that light is an electromagnetic wave, the properties that follow — transverse character, mutual perpendicularity of E and B, and the energy and momentum a wave carries — and the full electromagnetic spectrum with the uses of each band.

Displacement current

Maxwell's insight that a changing electric field acts as a current, resolving the inconsistency of Ampere's law in the gap of a charging capacitor.

Ampere-Maxwell law

Adds displacement current to conduction current in Ampere's law, making the set of electromagnetic equations complete and self-consistent.

Prediction of EM waves

Maxwell's equations predict self-sustaining waves travelling at 1/√(μ₀ε₀), a value matching the measured speed of light — establishing light as an EM wave.

Nature of EM waves

Oscillating electric and magnetic fields, mutually perpendicular and both perpendicular to the direction of propagation. No medium is needed.

Transverse character

Because E and B are perpendicular to the direction of travel, EM waves can be POLARISED — something impossible for a longitudinal wave like sound.

Speed of EM waves

In vacuum, 3 × 10⁸ m s⁻¹ for every wavelength. In a medium the speed drops to c/n, where n is the refractive index.

Ratio of E and B amplitudes

The two field amplitudes are locked together with E₀/B₀ = c, and the two fields oscillate exactly in phase.

Energy carried by an EM wave

Shared equally between the electric and magnetic fields. Intensity is proportional to the SQUARE of the electric field amplitude.

Momentum and radiation pressure

EM waves carry momentum U/c, so they exert pressure on a surface — the physics behind solar sails and comet tails.

Radio waves

The longest wavelengths, produced by accelerating charges in aerials. Used for radio, television and mobile communication.

Microwaves

Short-wavelength radio waves used in radar, satellite links and microwave ovens, where they resonate with water molecules to heat food.

Infrared radiation

Also called heat waves, emitted by all warm bodies. Used in remote controls, thermal imaging and night vision, and trapped by greenhouse gases.

Visible light

The narrow band from roughly 400 nm to 700 nm that the human eye detects — a tiny slice of the whole spectrum.

Ultraviolet radiation

Beyond violet, energetic enough to damage living tissue. The ozone layer absorbs most of the Sun's UV, which is why its depletion matters.

X-rays

Produced when fast electrons are suddenly stopped by a metal target. Used in medical imaging and crystallography for their high penetrating power.

Gamma rays

The shortest wavelength and highest energy, emitted by radioactive nuclei. Used in cancer therapy and in sterilising medical equipment.