The first evidence for electromagnetic waves, specifically light waves, can be attributed to the experiments conducted by Thomas Young in the early 19th century. Young's famous double-slit experiment provided compelling evidence that light behaves as a wave.
Young's experiment involved shining light through a barrier with two narrow slits and observing the resulting pattern on a screen placed behind the barrier. If light propagated only as particles (as Newton proposed), one would expect to see two distinct bands of light corresponding to the two slits on the screen. However, Young observed a pattern of alternating dark and light bands known as interference fringes.
This interference pattern could only be explained if light waves from each slit were overlapping and interfering with each other. The constructive and destructive interference of the light waves produced the observed pattern of bright and dark bands. Young's experiment provided strong evidence in support of the wave nature of light and the existence of electromagnetic waves.
Later advancements in the field of electromagnetism by scientists like James Clerk Maxwell and Heinrich Hertz further confirmed the existence of electromagnetic waves and laid the foundation for the development of the electromagnetic theory.