Newton's Prism Experiment

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

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.

At the time of his experiment the prevailing theory was that

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

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

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

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.

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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.

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

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

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

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

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.

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

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.

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

Okay that's pretty neat.

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.

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.

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.

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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.

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.