For most of the 19th century, electricity and magnetism were treated as separate fields. Static charges attracted lint. Magnets stuck to refrigerators. They had nothing to do with each other.

Then in 1820, the Danish physicist Hans Christian Ørsted noticed a compass needle deflecting near a current-carrying wire. The hint was clear: a moving electric charge creates a magnetic field. Faraday found the reverse — a moving magnet creates electric current. The two phenomena were tangled together.

In 1865, James Clerk Maxwell tied them up completely. He published four equations describing how electric and magnetic fields generate each other in space. When you crank through the math, the equations have a remarkable property: they predict that disturbances in the electromagnetic field travel as waves.

Maxwell calculated the speed of those waves from two physical constants — the strength of an electric field and the strength of a magnetic field — neither of which had anything obvious to do with light.

The number came out to 299,792 kilometers per second.

That was, to within experimental error, the measured speed of light. Maxwell wrote in his paper: 'The agreement of the results seems to show that light and magnetism are affections of the same substance.' Light wasn't its own thing. Light was an electromagnetic wave.

This unification rewired physics. Radio waves, microwaves, X-rays, gamma rays — all turned out to be the same kind of wave at different frequencies. The visible spectrum your eyes detect is a tiny window in a vast continuum.

Every WiFi router, every radio station, every X-ray machine works because of equations Maxwell wrote down before electric lights existed.