How was invisible electromagnetic radiation discovered?
The history of electromagnetism goes very far back – basically through observations of magnetic effects (800 BC, Greece).
The recognition of the forces as part of the electromagnetic spectrum of radiation may have started with James Clerk Maxwell.
In 1856 Maxwell developed the theory that the energy of the electromagnetic field is in the space around the conductors as well as in the conductors themselves. By 1864 he had formulated his own electromagnetic theory of light, predicting that both light and radio waves are electric and magnetic phenomena.
Maxwell predicted that electromagnetic disturbances traveling through empty space have electric and magnetic fields at right angles to each other and that both fields are perpendicular to the direction of the wave. He concluded that the waves move at a uniform speed equal to the speed of light and that light is one form of electromagnetic wave.
The Maxwell equations for the electromagnetic field unified all that was hitherto known about electricity and magnetism and predicted the existence of an electromagnetic phenomenon that can travel as waves.