 So Young used sunlight as his source and he put it through a little pinhole So he'd get a nice single point source of sunlight and then he put it through two other pinholes And he saw his interference fringes and from the placement of those He figured out that the wavelength of the red light was around about a micron and the wavelength The blue light was around about half that and the other main thing he showed was that light was clearly a wave Because you can't possibly get interference fringes if you don't have a wave You can't get two particles adding together to give you nothing or two particles adding together to give you If you equivalent of four particles, it has to be a wave if you're gonna have interference And so we went and published this and showed yes light as a wave. I've shown it and this of course made people very angry and so Kaposco theorists attacked him in the Edinburgh view and eventually he was sufficiently undermined and he went back and focused on medicine for a while in the years to come after that Fresnel wrote a thesis about wave properties of light and Poisson tried to prove him wrong by saying well obviously if lights are wave Then if you have a perfectly round Object then what you should have in the middle of the shadow is a bright spot because all the light going round the edges of Your blockage should all constructively interfere there and you get a bright spot now People originally took Poisson's suggestion as a disproof of the wave theory of light because no one had ever seen bright spots in the middle Of shadows before but in order to expect that bright spot You have to have your surface smooth on the scale of the wavelength of the wave and given that the wavelength of the wave was supposed To be around a micron it took them a while to make a ball that round and when they did of course What they saw was this bright spot now called Poisson spot and so our final conclusion was that light is certainly a wave