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In this video we are going to look at the relationship between white light and color by

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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.

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At the time of his experiment the prevailing theory was that

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white light was a color of light and that other colors could be created by modifing the white light some how.

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For instance this red piece of plastic would be described as changing this white light into red light.

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They also had knowledge about how light behaved at the boundary of 2 materials.

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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.

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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.

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Whether refractive index of the material is related to how fast light propagates through it.

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This expresstion allows us predict what will happen at plainer boundaries as we change angle for instance.

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It also allows us to deal with more complicated shapes like this triangular prism

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it's just a matter of geometry and keeping track of the angles.

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Newton was working on designing lenses for telescopes when he decided to investigate the phenomena prismatic colors.

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Those are the color that occur when you pass white light through a prism.

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So you obtained a triangular prism and you pass some white light through it and you saw a rainbow just as you expected.

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But then he noticed something in the direction that the colors were spread the pattern was wider than it should be.

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Based on the system geometry if light obeyed this fixed sign ratio law.

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So we did some experiments you separated out individual colors in the spectrum and passed them through additional prisms

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and you came to realize that all the colors in the spectrum are their own form of light and

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they all experience a different refractive index on traveling through these prisms.

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So this lead him to the conclusion that the white light entering the prism wasn't really white

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it was a combination of all these different colors and all the prism was doing was separating them in angle by varying refractive index.

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This is an interesting conclusion but doesn't really prove what's happening

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because we're still relying on this prism to make these colors.

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So what we really need is an experiment where we can form these colors from white light without a prism.

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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.

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We start with our screen close to the lense and we see the same spectrum we saw before,

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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.

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As we move our screen away the colors begin to overlap and until at one point we see a band of white light.

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As we continue to move the screen further away we see the same spectrum that we started with but with the colors now reversed.

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As we move the screen in this experiment there's nothing to cause this change of color we're observing

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the only thing that's changing is the overlap of the colors

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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.

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Now it turns out that you don't actualy need all these colors to trick your eyes into seeing white

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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.

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But for our purposes we're seeing the sum of all the colors in the input spectrum.

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Okay that's pretty neat.

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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.

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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.

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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.

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It turns out that answer is yes but it's a little bit more complicated than you would expect.

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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

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and to our eyes this looks it's working but it's not really,

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all that's really happening is the white hasn't had enough time to spread.

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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

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and you could see it more clearly by eye if we placed this prism further down.

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Over here it's clear that there is a change in color across the width of the beam.

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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.

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You start with a prism that we had before then you add a lense to the system

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and you want this lense to be roughly twice the focal length of the lense.

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At some distance away from the lense we will put another prism and this distance again should be roughly twice the focal length,

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and we adjust the prism and what we see is a reasonable proximation of white light.

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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

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and a very clear white beam at the output but for this video this will work.

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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.

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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.