Electromagnetic Waves
Class 12 Physics · Physics Part I · Chapter 8
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Summary
Maxwell completed the laws of electromagnetism by introducing the displacement current, \(I_d=\varepsilon_0\dfrac{d\Phi_E}{dt}\), recognising that a changing electric field acts as a source of magnetic field, just as a changing magnetic field produces an electric field. This symmetry predicts self-sustaining electromagnetic waves in which oscillating electric and magnetic fields, mutually perpendicular and both perpendicular to the direction of propagation, travel through vacuum at the speed of light \(c=\dfrac{1}{\sqrt{\mu_0\varepsilon_0}}\). The fields are in phase and related by \(c=E_0/B_0\); such waves are transverse and carry energy and momentum, exerting radiation pressure. Light itself is an electromagnetic wave, and the full electromagnetic spectrum—radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays—spans an enormous range of wavelengths and frequencies, all travelling at the same speed in vacuum but produced and detected by different means. Each region has characteristic sources, properties and uses, from radio communication and radar to medical imaging and sterilisation, unifying optics, electricity and magnetism into one grand framework.
Displacement currentMaxwell's synthesisNature of EM wavesEnergy and momentum of EM wavesElectromagnetic spectrumSources and uses of EM radiation
Key terms
Displacement current
Current-like term \(I_d=\varepsilon_0\,d\Phi_E/dt\) from a changing electric field.
Electromagnetic wave
Transverse wave of perpendicular oscillating \(\vec{E}\) and \(\vec{B}\) fields.
Speed of light
Wave speed in vacuum, \(c=1/\sqrt{\mu_0\varepsilon_0}\approx 3\times10^8\) m/s.
Radiation pressure
Pressure exerted by electromagnetic waves carrying momentum.
Electromagnetic spectrum
Ordered range of EM waves from radio to gamma rays.
Transverse wave
Wave whose oscillations are perpendicular to its direction of travel.
Extra questions & answers
Try one from this chapter
Maxwell introduced the displacement current to account for: