James Clerk Maxwell · Science
How Maxwell proved that light is nothing but a ripple in the electromagnetic field - unifying optics with electricity and magnetism, predicting an entire invisible spectrum of radiation, and changing forever what we mean by ‘light.’
What is light? It is among the oldest questions humans have asked, and for most of history the answers were guesses. The ancients thought sight was a ray leaving the eye. Newton held that light was a stream of tiny particles, corpuscles, flying through space. Huygens and later Young and Fresnel argued it was a wave, and by the early nineteenth century the wave theory had largely won, having explained how light bends around edges and produces coloured interference fringes. But a wave of what? Sound is a wave in air; ocean swells are waves in water. Light seemed to be a wave in nothing in particular, and no one could say what was actually waving.
Maxwell answered the question with stunning precision. Light, he showed, is a wave in the electromagnetic field - an oscillation of electric and magnetic fields, each generating the other, propagating through space. When light reaches your eye, what arrives is a rapidly vibrating electric field (and a magnetic field at right angles to it), oscillating millions of millions of times per second. The thing that waves is the field itself. After Maxwell, ‘what is light?’ had a real, quantitative answer for the first time: light is electromagnetism in motion. The science of optics, thousands of years old, had become a chapter of the theory of electricity and magnetism - a unification no one had seen coming.
Here is the most far-reaching consequence of Maxwell’s discovery. If light is an electromagnetic wave, then visible light - the narrow band of colours from red to violet that our eyes happen to detect - is only a tiny slice of a vast continuum. Maxwell’s equations place no special limit on the frequency of an electromagnetic wave. There can be waves vibrating slower than red light, and waves vibrating faster than violet, stretching away in both directions. These would be perfectly real radiation, identical in nature to visible light, differing from it in nothing but their frequency - and we would simply be blind to them.
This is the opening of the lesson. The rest — the dialogue, the primary source, and the recall — is in the app.
You learned that Maxwell identified light as an electromagnetic wave - oscillating electric and magnetic fields propagating through space at a speed his equations fixed. Explain why this unified optics with electromagnetism, and why it implied a whole spectrum of invisible radiation (radio, infrared, ultraviolet, X-ra…
Leads to Christiaan Huygens.
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