 And so to test whether light is a wave, what people did was they tried to see whether it would make an interference pattern. And Jung did this in the 1800s, and it turned out in order to get a nice interference pattern, he had to get the two sources really close together, so that they were more or less only a wavelength or two apart. And since the wavelength of light that we see is around about a micron or a millionth of a meter, that meant Jung had to get two very close slits together. And when he put the slits together, he saw regions of bright and dark light cast off from that, so he could tell, yes, I have an interference pattern, and therefore light is a wave. It took a few years for this idea that light was a wave to catch on. Thomas Jung only published his double slit experiment in 1807, and a decade later, the French Academy of Sciences was still unsure, and they launched a competition to explain the properties of light. Well, Gaston-Jean-Frenel put forth the theory that light was a wave, but one of the judges on the competition, Poisson, said that that was impossible because of the following idea. If you had a point source of light and you put a blockage in the way that was even, like a sphere, then every way that light would go around that sphere, if the wave went around the other side, then that would add up constructively because they'd go the same distance. And so all the ways that light could go around here would all be adding up constructively, and so if a light were a wave, what you should see is a bright spot in the middle of shadows. And he said, well, obviously there aren't bright spots in the middle of shadows, therefore the wave theory of light is wrong. Fortunately for science, the chair of the judging panel, Aragou, thought to actually do the experiment. Now to do the experiment, you have to make sure that the waves going on either side of the sphere go the same distance, and that means you have to make it smooth on the scale of the wavelength of light. And given that the wavelength of light is a micron, that meant he had to go to a lot of effort to make a blockage that was really, really smooth. And when he did that, what he saw was a bright spot in the middle of the shadow. So Frunel and Young suggested that light were a wave. Aragou actually proved that was true by showing the other spot in the middle, and then that spot is often called Poisson's spot. Even though Poisson was using it as an example of something that was obviously absurd and wouldn't exist, and therefore light wasn't a wave.