Newton's Prism Experiment - By MITK12Videos
Transcript
| 00:13 | In this video , we're going to look at the | |
| 00:14 | relationship between white light and color by recreating a portion | |
| 00:17 | of Newton's prism experiment is presented in a letter to | |
| 00:20 | the Royal Society in 1671 , but first , a | |
| 00:22 | little bit of background at the time of his experiment | |
| 00:26 | . The prevailing theory was that white light was the | |
| 00:28 | color of light and that other colors could be created | |
| 00:30 | by modifying the white light . Somehow , for instance | |
| 00:33 | , this red piece of plastic would be described as | |
| 00:35 | changing this white light into red light . They also | |
| 00:38 | had knowledge about how light behaves at the boundary between | |
| 00:40 | two materials . For instance , a planer boundary . | |
| 00:42 | They knew that the ratio of the sine of the | |
| 00:44 | angle on each side of the boundary was fixed for | |
| 00:47 | a given set of materials . We now know this | |
| 00:50 | is a form of Snell's law where the ratio of | |
| 00:53 | the signs of the angles is equal to the inverse | |
| 00:55 | ratio of the refractive indices of the materials where the | |
| 00:57 | refractive index of the materials related to how fast light | |
| 01:00 | propagates through it . This expression allows us to predict | |
| 01:04 | what will happen to the plane or boundary as we | |
| 01:05 | change the angle of instance . It also allows us | |
| 01:08 | to deal with more complicated shapes like this triangular prism | |
| 01:11 | . It's just a matter of geometry and keeping track | |
| 01:14 | of the angles . Yeah , it was working on | |
| 01:16 | designing lenses for telescopes , we decided to investigate the | |
| 01:19 | phenomena prismatic colors . Those are the colors that occur | |
| 01:22 | when you pass white light through a prism , so | |
| 01:24 | obtained a triangular prism and he passed some white light | |
| 01:28 | through it and he saw rainbow just like he expected | |
| 01:32 | . But then he knows something in the direction that | |
| 01:35 | the colors were spread . The pattern was much wider | |
| 01:37 | than it should be based on the system geometry if | |
| 01:40 | light obeyed this fixed sign ratio law . So we | |
| 01:44 | did some experiments , he separated out individual colors in | |
| 01:47 | the spectrum and pass them through additional prisms . And | |
| 01:49 | when I came to realize was that all the colors | |
| 01:51 | in the spectrum are their own form of light , | |
| 01:54 | and they all experienced a different refractive index on traveling | |
| 01:57 | through these prisms . So this led him to the | |
| 01:59 | conclusion that the white light entering the prison wasn't really | |
| 02:04 | white . It was a combination of all these different | |
| 02:06 | colors and that all the prison was doing was separating | |
| 02:09 | them an angle by this varying refractive index . This | |
| 02:13 | is an interesting conclusion , but doesn't really prove what's | |
| 02:16 | happening because we're still relying on this prism to make | |
| 02:19 | these colors . So what we really need is an | |
| 02:21 | experiment where we can form these colors from white light | |
| 02:24 | without a prism . And at the end of his | |
| 02:26 | paper , Newton suggests just such an experiment . You | |
| 02:30 | start with the same system you had before and then | |
| 02:32 | you place a lens in the system . We start | |
| 02:40 | with our screen close to the lens and we see | |
| 02:41 | the same spectrum we saw before . Here is the | |
| 02:44 | light passing through the lens and above it we see | |
| 02:46 | the light that's sort of skipping the top of the | |
| 02:48 | lens As we move our screen away , the colors | |
| 02:51 | begin to overlap , until at one we see a | |
| 02:54 | band of white light as we continue to move the | |
| 02:57 | screen further away , we see the same spectrum that | |
| 03:00 | we started with , but with the colours now reversed | |
| 03:02 | as we move the screen in this experiment , there's | |
| 03:05 | nothing to cause this change of color that we're observing | |
| 03:08 | . The only thing that's changing is the overlap of | |
| 03:10 | the colors . So we can conclude that when we | |
| 03:12 | perceive this white light , what we're really seeing is | |
| 03:15 | a whole bunch of colours added together . Now it | |
| 03:19 | turns out that you don't actually need all these colors | |
| 03:21 | to trick your eyes into seeing white . If you're | |
| 03:23 | watching this on tv screen or a computer screen at | |
| 03:25 | home , what you're seeing is white is actually a | |
| 03:28 | combination of red , blue and green . But for | |
| 03:30 | our purposes we're seeing the sum of all the colors | |
| 03:34 | in the input spectrum . Okay , that's pretty neat | |
| 03:37 | . We start off with white light , we form | |
| 03:39 | a spectrum of color and then we use lens to | |
| 03:41 | combine it back in the white light but it only | |
| 03:44 | really combines in the white light at one spot . | |
| 03:47 | If we go further away from the lens and closer | |
| 03:49 | to the lens it's still clearly a spectrum . So | |
| 03:51 | is there a way to combine this white light so | |
| 03:53 | that we get a beam of white light , sort | |
| 03:55 | of like we had at the input . It turns | |
| 03:57 | out the answer is yes , but it's a little | |
| 03:59 | bit more complicated than you would expect . So a | |
| 04:03 | lot of books draw this system where we start with | |
| 04:05 | our original prison and we put a second one in | |
| 04:08 | something like this . And to our eyes , this | |
| 04:11 | looks like it's working . But it's not really all | |
| 04:14 | that's really happening is the light hasn't had enough time | |
| 04:18 | to spread , so it looks like it's white . | |
| 04:21 | But if you had a very sensitive instrument , you'd | |
| 04:22 | be able to tell that there's a change in color | |
| 04:24 | across this and you can see it more clearly by | |
| 04:27 | . I if we place this prism further down over | |
| 04:32 | here , it's clear that there's a change in color | |
| 04:34 | across the width of the beam . If you really | |
| 04:38 | want to make a beam of white light from this | |
| 04:39 | coloured spectrum , you can follow the method outlined in | |
| 04:41 | Newton's optics . This comes from his last experiment in | |
| 04:44 | book one . You start with a prism that we | |
| 04:47 | had before . Then you add the lens to the | |
| 04:49 | system and you want this length to be roughly twice | |
| 04:52 | the focal length of the lens at some distance away | |
| 04:57 | from the lens , will put another prison and this | |
| 05:00 | distance again should be roughly twice the focal length and | |
| 05:03 | we adjust the prison , and what we see is | |
| 05:08 | a reasonable approximation of white light . Now , you | |
| 05:11 | really should build this system with a much larger focal | |
| 05:14 | length lens and should build a much wider system to | |
| 05:16 | get a really good separation between these colors here and | |
| 05:19 | a very clear white beam at the output . But | |
| 05:22 | for this video , this will work . Thank you | |
| 05:26 | for watching . I hope you found this material interesting | |
| 05:28 | . If you'd like to learn more about Newton's optics | |
| 05:30 | experiments , I'd recommend to Resources online . One is | |
| 05:34 | the Project Gutenberg , where you can find a copy | |
| 05:36 | of Newton's book Optics . And the other is the | |
| 05:39 | Newton Project , where you can find a copy of | |
| 05:41 | most of Newton's papers . |
Summarizer
DESCRIPTION:
OVERVIEW:
Newton's Prism Experiment is a free educational video by MITK12Videos.
This page not only allows students and teachers view Newton's Prism Experiment videos but also find engaging Sample Questions, Apps, Pins, Worksheets, Books related to the following topics.
GRADES:
STANDARDS:
