Oh, there’s a beautiful experiment first done back in 1818 that really was the decisive experiment in favor of treating light as a wave: The infamous Arago spot.
You see, in the early 19th century, whether or not light traveled in a straight line or behaved more like a wave was an open question. Sure, there were experiments like Young’s, but they were not considered decisive. Newton’s corpuscular theory of light was still the preferred view by many.
Then came Poisson, who studied Fresnel’s wave theory of light, in particular the diffraction of light by the edges of objects. Poisson calculated that when it comes to the shadow of a circular object, diffraction should cause a faint spot of light appear at the exact center of its circular shadow. This faint spot would be there because light waves, diffracted by the edges of the circular object, would arrive at the center (and only at the center) with the same phase, causing constructive interference. Elsewhere, rays of light will have traveled paths of different lengths from different parts of the circumference of the circular object, and thus no constructive interference would occur.
Poisson actually thought that this nonsensical prediction (a spot of light at the center of the shadow) would disprove the wave theory of light. However, the head of the committee entrusted to study the properties of light by the French Academy of Sciences, Arago, behaved exactly like a proper scientist should: he actually carried out the experiment!
And guess what: The Arago spot was there. It is not easy to see, e.g., in this Wikipedia picture (supposedly an actual photo, not a simulated image), but it’s right there in the center, I can see it even on my mediocre quality Laptop:
Because of its convoluted history, that faint spot at the center is sometimes called the Arago spot, the Fresnel-Arago spot, even the Poisson-Arago spot. Whatever we call it, it was the decisive experiment that proved the wave nature of light.
In the decades that followed, light was of course eventually unified with electromagnetic phenomena in the form of Maxwell’s theory, and then came the quantum theory, then quantum electrodynamics, yielding our modern view of the electromagnetic field as a quantum field, its quantized excitations serving as light quanta, i.e., photons, which sometimes behave just like Newton’s light corpuscles were expected to behave. But that, as they say, is a story for another day.
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