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How did James Clerk Maxwell’s theory connect light with electromagnetism?

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A horizontal gradient bar displays the visible light spectrum from violet to red, labeled with corresponding frequency in terahertz, wavelength in nanometers, and photon energy in electron volts. — light When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.

James Clerk Maxwell unified the fields of electricity, magnetism, and optics in the 1860s by demonstrating that light is a form of electromagnetic radiation.1

Building on Michael Faraday’s observations, Maxwell formulated a set of partial differential equations describing how changing electric fields produce magnetic fields and vice versa. He realized that this reciprocal relationship allows for the propagation of self-sustaining electromagnetic waves.234

Crucially, Maxwell calculated the speed of these theoretical waves using constants from electrostatic and magnetostatic laws. The resulting value—approximately $3 \times 10^{8}$ metres per second—matched the known speed of light. This led him to conclude that light is an electromagnetic disturbance propagating through the field. His work transformed the study of light into a branch of electromagnetism, providing a foundational model that remains essential in modern physics.41

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