[00:01] slit interference but have you ever thought about exactly what it is why it you don't observe it with things like white light right here is a nice physical demo at the Exploratorium where a laser is shot through two very thin
[00:16] slits and down on a wall several feet away from the slits you don't just see two bright spots you see many let's go ahead and pull up a simplified what's happening if the slits are thin enough you can model each one of them as
[00:30] what follows for the light coming from each one to be a pure sine wave with just a single wavelength think about a point on the middle of the wall behind these slits the distance to each slit is the same so those two waves are in Phase
[00:44] with each other and this gives what we call constructive interference so you see a bright spot but if you shift a little to the side to a point such that the distance to one slit is exactly half a wavelength longer than the distance to
[00:56] the other then the two waves actually cancel out the peaks of one line up with the troughs of the other the net result is nothing and you get a dark spot when you think about it this actually feels really bizarre at first because it means
[01:09] when you go from having one slit to opening a second this specific point of the wall will actually become darker despite more Total light being let consequence of the fact that you're adding waves similarly if you scan from
[01:23] left to right these waves fall in and out of sync with each other giving oscillations between bright and dark spots but but that specific pattern depends a lot on the specific wavelength of the light it changes if you change
[01:35] that wavelength this is why if you tried it with white light which is a mix of a lot of different pure sine waves you wouldn't see the distinctive pattern other which is why it took until 1800 for anyone to notice this phenomenon