Introduction

Every chapter so far in this Semester -- electrostatics, current electricity, the magnetic effects of current, and electromagnetic induction -- has treated electric and magnetic fields as, in a sense, separate phenomena that happen to influence one another: a current produces a magnetic field (Oersted), a changing magnetic flux induces an electric field/emf (Faraday). This chapter completes the picture James Clerk Maxwell assembled around 1864-65 by adding the one missing symmetry -- that a changing ELECTRIC field can, on its own, also act as a source of a MAGNETIC field -- and shows that once this symmetry is in place, the complete set of equations governing electricity and magnetism predicts something genuinely new: a self-propagating disturbance of electric and magnetic fields, an electromagnetic wave, that requires no material medium whatsoever to travel and that moves, in vacuum, at an exact, calculable speed -- which Maxwell found to match the already-measured speed of light so closely that he correctly concluded light ITSELF must be an electromagnetic wave.

How this unit is organised. Section 8.2 introduces displacement current, the missing term Maxwell added to Ampère's circuital law, using the charging parallel-plate capacitor as the standard illustration of why the older law, unmodified, gives a contradiction. Section 8.3 identifies what physically sets an electromagnetic wave going -- an accelerating (typically oscillating) electric charge. Section 8.4 works out the wave's transverse character: its electric field, its magnetic field, and its direction of travel are all mutually perpendicular. Section 8.5 gives the wave's speed in vacuum, $c=1/\sqrt{\mu_0\epsilon_0}$, built purely from the permeability and permittivity of free space met in earlier chapters. Section 8.6 looks qualitatively at how the wave carries energy as it travels. Section 8.7 closes the unit with the full electromagnetic spectrum -- radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays -- each band differing from its neighbours only in wavelength/frequency, with its own typical method of production and everyday use, exactly as WBCHSE's own syllabus text asks for.