Physics books often state that scientists believed light to be a particle (as Newton said) until Thomas Young proved it to be a wave with his famous two-slit (actually, two pinhole) experiment. That's a nice story, but it didn't exactly work that way.
The wave theory had its proponents even in Newton's day, but they were largely ignored. In his classic text, Optics, 3rd Ed (Addison Wesley Longman, 1998) Eugene Hecht says "The great weight of Newton's opinion hung like a shroud over the wave theory during the 18th century, all but stifling its advocates."
Thomas Young offered papers before the Royal Society in 1801, 1802 and 1803, presenting the "principle of interference" to support the wave theory of light and his observations. There wasn't immediate acceptance of his ideas. In fact, one reviewer said Young's papers were "destitute of every species of merit" (again, from Hecht's book).
At the same time that Young was trying to convince his English colleagues of the wave nature of light, Augustin Fresnel was working on optical wave theory in France. In 1818, Fresnel submitted a paper on the wave theory of light to a competition of the French Academy. Simeon Poisson, one of the judges, pointed out a ridiculous prediction of Fresnel's theory: light passing around a solid obstacle, such as a ball bearing, would cast a shadow with a bright spot at the center. This was clearly ludicrous! When another of the judges, Dominique Arago, performed the experiment that showed the bright spot, Fresnel was named winner of the competition. Ironically, the bright spot in the center of the dark shadow is usually called "Poisson's spot" after the scientist who doubted its existence.
At this time (early 1800s) the wave theory proponents believed that light is a longitudinal wave, like sound. Light was seen as "pressure" variations traveling through an invisible "aether". The discovery of the polarization of reflected light put the wave theory in doubt, and Young, Fresnel and Arago worked feverishly on the problem until Young suggested that light is a transverse wave, vibrating at right angles to the direction of travel. Polarization is then easily explained by restricting light to vibrate in only one direction.
It was not until 1825 that the wave theory was very well established. While Young's experiment was the starting point of the new wave theory, it took 25 years before Newton's "corpuscles" of light were overthrown.