CBSE Class 12 Board Examination

Chapter 8: Electromagnetic Waves

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.

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