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Dialogue: 0,0:00:12.36,0:00:19.73,Default,,0000,0000,0000,,In this video we are going to look at the relationship between white light and color by
Dialogue: 0,0:00:19.73,0:00:25.55,Default,,0000,0000,0000,,recreating a portion of Newton's prism experiment as presented in a letter to the royal society in 1671 but first a little bit of background.
Dialogue: 0,0:00:25.55,0:00:28.95,Default,,0000,0000,0000,,At the time of his experiment the prevailing theory was that
Dialogue: 0,0:00:28.95,0:00:33.29,Default,,0000,0000,0000,,white light was a color of light and that other colors could be created by modifing the white light some how.
Dialogue: 0,0:00:33.29,0:00:38.64,Default,,0000,0000,0000,,For instance this red piece of plastic would be described as changing this white light into red light.
Dialogue: 0,0:00:38.64,0:00:42.35,Default,,0000,0000,0000,,They also had knowledge about how light behaved at the boundary of 2 materials.
Dialogue: 0,0:00:42.35,0:00:50.26,Default,,0000,0000,0000,,For instance a plainer boundary they knew that the ratio of the sign of the angle on each side of the boundary was fixed forgetting some of the materials.
Dialogue: 0,0:00:50.26,0:00:58.17,Default,,0000,0000,0000,,We now know this is a form of Snell's law where the ratio of the signs of the angles is equal to the inverse ratio of their refractive indices of the materials.
Dialogue: 0,0:00:58.17,0:01:03.58,Default,,0000,0000,0000,,Whether refractive index of the material is related to how fast light propagates through it.
Dialogue: 0,0:01:03.58,0:01:08.34,Default,,0000,0000,0000,,This expresstion allows us predict what will happen at plainer boundaries as we change angle for instance.
Dialogue: 0,0:01:08.34,0:01:12.75,Default,,0000,0000,0000,,It also allows us to deal with more complicated shapes like this triangular prism
Dialogue: 0,0:01:12.75,0:01:16.84,Default,,0000,0000,0000,,it's just a matter of geometry and keeping track of the angles.
Dialogue: 0,0:01:16.84,0:01:22.09,Default,,0000,0000,0000,,Newton was working on designing lenses for telescopes when he decided to investigate the phenomena prismatic colors.
Dialogue: 0,0:01:22.09,0:01:26.70,Default,,0000,0000,0000,,Those are the color that occur when you pass white light through a prism.
Dialogue: 0,0:01:26.70,0:01:33.13,Default,,0000,0000,0000,,So you obtained a triangular prism and you pass some white light through it and you saw a rainbow just as you expected.
Dialogue: 0,0:01:33.57,0:01:40.05,Default,,0000,0000,0000,,But then he noticed something in the direction that the colors were spread the pattern was wider than it should be.
Dialogue: 0,0:01:40.05,0:01:44.32,Default,,0000,0000,0000,,Based on the system geometry if light obeyed this fixed sign ratio law.
Dialogue: 0,0:01:44.32,0:01:51.02,Default,,0000,0000,0000,,So we did some experiments you separated out individual colors in the spectrum and passed them through additional prisms
Dialogue: 0,0:01:51.02,0:01:54.59,Default,,0000,0000,0000,,and you came to realize that all the colors in the spectrum are their own form of light and
Dialogue: 0,0:01:54.59,0:01:59.75,Default,,0000,0000,0000,,they all experience a different refractive index on traveling through these prisms.
Dialogue: 0,0:01:59.75,0:02:05.69,Default,,0000,0000,0000,,So this lead him to the conclusion that the white light entering the prism wasn't really white
Dialogue: 0,0:02:05.69,0:02:14.27,Default,,0000,0000,0000,,it was a combination of all these different colors and all the prism was doing was separating them in angle by varying refractive index.
Dialogue: 0,0:02:14.27,0:02:18.02,Default,,0000,0000,0000,,This is an interesting conclusion but doesn't really prove what's happening
Dialogue: 0,0:02:18.02,0:02:21.70,Default,,0000,0000,0000,,because we're still relying on this prism to make these colors.
Dialogue: 0,0:02:21.70,0:02:26.56,Default,,0000,0000,0000,,So what we really need is an experiment where we can form these colors from white light without a prism.
Dialogue: 0,0:02:26.56,0:02:40.71,Default,,0000,0000,0000,,And at the end of this paper Newton suggests such an experiment you start with the same system you had before and then you place a lense in the system.
Dialogue: 0,0:02:40.71,0:02:44.30,Default,,0000,0000,0000,,We start with our screen close to the lense and we see the same spectrum we saw before,
Dialogue: 0,0:02:44.30,0:02:50.31,Default,,0000,0000,0000,,here's the light passing through the lense and up above that we see the light that's sort of skipping the top of the lense.
Dialogue: 0,0:02:50.31,0:02:57.14,Default,,0000,0000,0000,,As we move our screen away the colors begin to overlap and until at one point we see a band of white light.
Dialogue: 0,0:02:57.14,0:03:03.85,Default,,0000,0000,0000,,As we continue to move the screen further away we see the same spectrum that we started with but with the colors now reversed.
Dialogue: 0,0:03:03.85,0:03:08.94,Default,,0000,0000,0000,,As we move the screen in this experiment there's nothing to cause this change of color we're observing
Dialogue: 0,0:03:08.94,0:03:12.67,Default,,0000,0000,0000,,the only thing that's changing is the overlap of the colors
Dialogue: 0,0:03:12.67,0:03:19.76,Default,,0000,0000,0000,,so we can conclude that when we've perceived this white light what we're really seeing is a whole bunch of colors added together.
Dialogue: 0,0:03:19.76,0:03:24.04,Default,,0000,0000,0000,,Now it turns out that you don't actualy need all these colors to trick your eyes into seeing white
Dialogue: 0,0:03:24.04,0:03:30.62,Default,,0000,0000,0000,,if you're watching this on a TV screen or a computer screen at home what you are seeing as white is actually a combination of red blue and green.
Dialogue: 0,0:03:30.62,0:03:37.05,Default,,0000,0000,0000,,But for our purposes we're seeing the sum of all the colors in the input spectrum.
Dialogue: 0,0:03:37.05,0:03:38.90,Default,,0000,0000,0000,,Okay that's pretty neat.
Dialogue: 0,0:03:38.90,0:03:45.11,Default,,0000,0000,0000,,We start off with white light we form a spectrum of color and then we use a lense to combine it back into white light.
Dialogue: 0,0:03:45.11,0:03:52.72,Default,,0000,0000,0000,,But it only really combines it into white light at one spot if we go further away from the lense and closer to the lense it's still clearly a spectrum.
Dialogue: 0,0:03:52.72,0:03:58.09,Default,,0000,0000,0000,,So is there a way to combine this white light so we get a beam of white light sort of like we had at the input.
Dialogue: 0,0:03:58.09,0:04:03.81,Default,,0000,0000,0000,,It turns out that answer is yes but it's a little bit more complicated than you would expect.
Dialogue: 0,0:04:03.81,0:04:10.96,Default,,0000,0000,0000,,So a lot of books draw this system where we start with our original prism and we put a second one in something like this
Dialogue: 0,0:04:10.96,0:04:15.91,Default,,0000,0000,0000,,and to our eyes this looks it's working but it's not really,
Dialogue: 0,0:04:15.91,0:04:20.84,Default,,0000,0000,0000,,all that's really happening is the white hasn't had enough time to spread.
Dialogue: 0,0:04:20.84,0:04:26.78,Default,,0000,0000,0000,,So it looks like it's white but if you had a very sensitive instrument you would be able to tell that there is a changing color across this
Dialogue: 0,0:04:26.78,0:04:31.77,Default,,0000,0000,0000,,and you could see it more clearly by eye if we placed this prism further down.
Dialogue: 0,0:04:31.77,0:04:38.16,Default,,0000,0000,0000,,Over here it's clear that there is a change in color across the width of the beam.
Dialogue: 0,0:04:38.16,0:04:47.54,Default,,0000,0000,0000,,If you really want to make a beam of white light from this colored spectrum you can follow the method outlined in Newton's optics this comes from his last experiment in book 1.
Dialogue: 0,0:04:47.54,0:04:51.17,Default,,0000,0000,0000,,You start with a prism that we had before then you add a lense to the system
Dialogue: 0,0:04:51.17,0:04:57.48,Default,,0000,0000,0000,,and you want this lense to be roughly twice the focal length of the lense.
Dialogue: 0,0:04:57.48,0:05:04.18,Default,,0000,0000,0000,,At some distance away from the lense we will put another prism and this distance again should be roughly twice the focal length,
Dialogue: 0,0:05:04.18,0:05:12.46,Default,,0000,0000,0000,,and we adjust the prism and what we see is a reasonable proximation of white light.
Dialogue: 0,0:05:12.46,0:05:20.52,Default,,0000,0000,0000,,Now you really should build this system with a much larger focal length lense and should build a much wider system to get a really good speration between these colors here
Dialogue: 0,0:05:20.52,0:05:26.58,Default,,0000,0000,0000,,and a very clear white beam at the output but for this video this will work.
Dialogue: 0,0:05:26.58,0:05:34.87,Default,,0000,0000,0000,,Thank you for watching I hope you found this material interesting if you would like to learn more about Newton's optics experiments I recommend 2 resorses online.
Dialogue: 0,0:05:34.87,0:05:46.61,Default,,0000,0000,0000,,One is the Project Gutenberg where you can find a copy of Newton's book optics and the other is The Newton's Project where you can find a copy of most of Newton's papers.