WEBVTT 00:00:12.361 --> 00:00:19.727 In this video we are going to look at the relationship between white light and color by 00:00:19.727 --> 00:00:25.553 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. 00:00:25.553 --> 00:00:28.947 At the time of his experiment the prevailing theory was that 00:00:28.947 --> 00:00:33.291 white light was a color of light and that other colors could be created by modifing the white light some how. 00:00:33.291 --> 00:00:38.636 For instance this red piece of plastic would be described as changing this white light into red light. 00:00:38.636 --> 00:00:42.346 They also had knowledge about how light behaved at the boundary of 2 materials. 00:00:42.346 --> 00:00:50.256 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. 00:00:50.256 --> 00:00:58.170 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. 00:00:58.170 --> 00:01:03.583 Whether refractive index of the material is related to how fast light propagates through it. 00:01:03.583 --> 00:01:08.335 This expresstion allows us predict what will happen at plainer boundaries as we change angle for instance. 00:01:08.335 --> 00:01:12.746 It also allows us to deal with more complicated shapes like this triangular prism 00:01:12.746 --> 00:01:16.837 it's just a matter of geometry and keeping track of the angles. 00:01:16.837 --> 00:01:22.089 Newton was working on designing lenses for telescopes when he decided to investigate the phenomena prismatic colors. 00:01:22.089 --> 00:01:26.704 Those are the color that occur when you pass white light through a prism. 00:01:26.704 --> 00:01:33.127 So you obtained a triangular prism and you pass some white light through it and you saw a rainbow just as you expected. 00:01:33.569 --> 00:01:40.053 But then he noticed something in the direction that the colors were spread the pattern was wider than it should be. 00:01:40.053 --> 00:01:44.317 Based on the system geometry if light obeyed this fixed sign ratio law. 00:01:44.317 --> 00:01:51.022 So we did some experiments you separated out individual colors in the spectrum and passed them through additional prisms 00:01:51.022 --> 00:01:54.592 and you came to realize that all the colors in the spectrum are their own form of light and 00:01:54.592 --> 00:01:59.752 they all experience a different refractive index on traveling through these prisms. 00:01:59.752 --> 00:02:05.693 So this lead him to the conclusion that the white light entering the prism wasn't really white 00:02:05.693 --> 00:02:14.274 it was a combination of all these different colors and all the prism was doing was separating them in angle by varying refractive index. 00:02:14.274 --> 00:02:18.023 This is an interesting conclusion but doesn't really prove what's happening 00:02:18.023 --> 00:02:21.702 because we're still relying on this prism to make these colors. 00:02:21.702 --> 00:02:26.563 So what we really need is an experiment where we can form these colors from white light without a prism. 00:02:26.563 --> 00:02:40.713 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. 00:02:40.713 --> 00:02:44.303 We start with our screen close to the lense and we see the same spectrum we saw before, 00:02:44.303 --> 00:02:50.313 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. 00:02:50.313 --> 00:02:57.144 As we move our screen away the colors begin to overlap and until at one point we see a band of white light. 00:02:57.144 --> 00:03:03.846 As we continue to move the screen further away we see the same spectrum that we started with but with the colors now reversed. 00:03:03.846 --> 00:03:08.941 As we move the screen in this experiment there's nothing to cause this change of color we're observing 00:03:08.941 --> 00:03:12.668 the only thing that's changing is the overlap of the colors 00:03:12.668 --> 00:03:19.755 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. 00:03:19.755 --> 00:03:24.039 Now it turns out that you don't actualy need all these colors to trick your eyes into seeing white 00:03:24.039 --> 00:03:30.623 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. 00:03:30.623 --> 00:03:37.047 But for our purposes we're seeing the sum of all the colors in the input spectrum. 00:03:37.047 --> 00:03:38.902 Okay that's pretty neat. 00:03:38.902 --> 00:03:45.107 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. 00:03:45.107 --> 00:03:52.722 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. 00:03:52.722 --> 00:03:58.092 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. 00:03:58.092 --> 00:04:03.812 It turns out that answer is yes but it's a little bit more complicated than you would expect. 00:04:03.812 --> 00:04:10.963 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 00:04:10.963 --> 00:04:15.911 and to our eyes this looks it's working but it's not really, 00:04:15.911 --> 00:04:20.839 all that's really happening is the white hasn't had enough time to spread. 00:04:20.839 --> 00:04:26.776 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 00:04:26.776 --> 00:04:31.773 and you could see it more clearly by eye if we placed this prism further down. 00:04:31.773 --> 00:04:38.160 Over here it's clear that there is a change in color across the width of the beam. 00:04:38.160 --> 00:04:47.535 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. 00:04:47.535 --> 00:04:51.170 You start with a prism that we had before then you add a lense to the system 00:04:51.170 --> 00:04:57.485 and you want this lense to be roughly twice the focal length of the lense. 00:04:57.485 --> 00:05:04.177 At some distance away from the lense we will put another prism and this distance again should be roughly twice the focal length, 00:05:04.177 --> 00:05:12.459 and we adjust the prism and what we see is a reasonable proximation of white light. 00:05:12.459 --> 00:05:20.519 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 00:05:20.519 --> 00:05:26.579 and a very clear white beam at the output but for this video this will work. 00:05:26.579 --> 00:05:34.870 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. 00:05:34.870 --> 00:05:46.608 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.