Electromagnetic Waves

Bringing the previous section's four equations together, an electromagnetic wave is the coupled, self-sustaining disturbance of electric and magnetic fields that those equations predict must exist. Two defining properties follow immediately from Maxwell's equations and from Hertz's experimental confirmation of them: an electromagnetic wave is non-mechanical, meaning it requires no material medium at all to travel through (unlike sound, which needs air, or a wave on a string, which needs the string) -- it propagates equally well through the vacuum of outer space as through glass or air; and it is a transverse wave, meaning the oscillating quantities (the electric and magnetic fields) vibrate perpendicular to the direction the wave travels in, rather than along it. In vacuum, every electromagnetic wave -- regardless of its frequency or wavelength -- travels at exactly the same speed, the speed of light, $c=1/\sqrt{\mu_0\epsilon_0}\approx3\times10^8$ m/s. The following subsections build out this picture in detail: how such waves are actually produced and what their full list of properties is (5.2.1), what physical situations act as sources of ele …