Polarized Light

hey guys I'm Kevin and today I'd like to share with you some of my favorite experiments and they have to do with polarization so what exactly is polarization well white is an electromagnetic wave and so it has an electric component that oscillates that goes up comes back down and keeps doing that as it moves through time and also as it moves through space but it's electromagnetic so it's got this electric component but it also has a magnetic component and this does the same thing it oscillates up and down and back now they go together and

so we have some they go together in a very particular way in fact the magnetic component is exactly perpendicular at right angles to the electric component so it looks something like this as it's going through space so you can see the magnetic electric Fields oscillate together they're out of phase so this light is linearly polarized because the electric field just goes in One Direction it goes straight up and down now if we we're say like this that's going to be vertical polarization and we move it this way we call that this way we call that

horizontal polarization now light doesn't have to be just vertically polarized or just horizontally polarized it can be anywhere in between light sources that produce light that's polarized all all over the place what we call unpolarized light so this is a flashlight and it's going to produce unpolarized light and I can demonstrate this for you by using a polarizer so this polarizer here is labeled axis and then it has a little arrow and that shows you which way it lets through or which polarization it lets through it um so the first thing you notice is that

this is somewhat dimmer than the backgrounds and that's because it's not letting all the light through it it's only letting the light that's polarized in this case uh with the electric field horizontal so if we look at our light source through this it's dimmer but it's still there now we can rotate it around and again light still getting through so we have light that's getting through that's both vertically polarized horizontally polarized and everywhere in between now if we expand this and we take another polarizer just the same again now we've got the axis horizontal we

can have light pass through one polarizer and the other one so we put them this way you see all the light gets through and that's because after it goes through this one it's horizontally polarized and that horizontally polarized light gets through makes through the second one now if we rotate these so that one of them is perpendicular to the other one now all of a sudden no light's getting through that's because the light that comes through this one first just show you how these are set up like that is all horizontally polarized but this one

only lets through vertically polarized light so as a result no light gets through do this with our flashlight again okay now I said we're going to use polarized sunglasses so first of all let's find out how they're polarized we'll get and use one of our polarizers that we know which way is polarized and put it here in front of the light that's producing horizontally polarized light and we can start with our sunglasses start rotating them around you see the light Mak it through there it gets pretty dim starts getting brighter again like that that's maybe

the brightest and then back around so it's Diest right about here so which way does that mean that the sunglasses are going to have to be polarized it's right vertically so we can go ahead and Mark our sunglasses as vertical polarizers here now let's do something interesting so we have our sunglasses um we have our initial polarizer here this is horizontally po polarized sunglasses are vertically polarized more or less there we go I think that's the best I can do what happens if I use another polarizer but I stick it in instead of well if

I stick it in horizontally it's about the same if I stick it in vertically well it eliminates that light there what happens if I stick it in an angle let's find out oh actually gets brighter that's interesting what's going on there so that was kind of interesting what's going on there well let's take a look at it step by step so we have our flashlight first which produces unpolarized light so we've got some vertical some horizontal side to side and then we pass it through a polarizer which only lets through horizontally polarized light then if

we had what we call the analyzer the sunglasses just vertically polarized that would block all the light but we don't we have this other polarizer set at an angle so when this light arrives at the polarizer it's horizontally polarize I'll draw that as a black line here but then the polarizer at an angle like this so that's going to let through light that's at this angle as well and so some of this light that arrives it is going to get repolarized along this diagonal now similarly we come to our analyzer pair of sunglasses here they're

polarized this way the light that arrives is diagonal and again there's some component of that that is now vertical and that makes it through to our eyes so that's now why we see an increase from what was previously zero light there well that's kind of neat but you could be asking yourself is there an application for this and there sure is it's in liquid Crystal displays so the way a liquid crystal display works is it has a back pane that's illuminated from The Source behind it this pan is polarized again and it has a front

pane on it that's cross-polarized so normally no light gets through um however what they do then is they put in some liquid Crystal what that Liquid Crystal is is it's kind of rodlike material and it lines itself naturally here with the polarization axis and then it rotates slowly up to this front analyzer axis and so in that case um what we actually have is as it's rotating it's rotating the light with it so it's as if we were having this angled polarizer in there and we're getting some light through when we apply a voltage though

the rods they all shift and so they go flat and it's as if we removed it and we get something dark so where we use LCD screens uh pretty much everywhere we have them on our calculators and of course also on our computers so here I pulled up just a blank PowerPoint slide and I'm going to show you that the monitor the light coming out of the monitor it's polarized because it has that uh final analyzer on it so we've got our polarizer here put on our polarized sunglasses and we can rotate them around and

you see that and we block out most of the light and it comes back so the light coming out is polarized and that's actually pretty well polarized too so we'll leave them on and let's do something cool with this we're actually I'm going to take this off from it to show you um we're going to look at the stress in this petri dish so if we just look at the petri dish then yeah I mean it looks okay we kind of maybe see something at an angle if we put on our analyzer and then look

at the petri dish whoa now we get a bunch of bright colors so what's going on there any ideas as the light is passing through the petri dish it's getting rotated just as it was passing through the rotating Liquid Crystal and that lets more and less light through and since the p dish is very and even and even some areas of it pass through for example more red than blue and that gives us this nice rainbow effect um it's also very striking of course if we rotate the analyzer so we're not seeing a whole lot

of light and then we put it in there too now if you're wearing your polarized sunglasses you're probably outside and and one of the things you might notice is that at the sky changes its Darkness as you change your head so right now I've got the camera without any polarizers on it and let's take a look at this so here's our polarizer with the axis labeled again it's pointing this way so let's move this around and then if we rotate it no that it gets brighter so it's letting more light through this way and then

darker if I rotate it this way so in fact the light coming from this part of the sky is vertically polarized now is that the same for all parts of the sky not entirely that one's actually doing pretty good over there but over here it's about the same now why is that so the sun is over there the sun's emitting light towards us and that light is unpolarized but then when it scatters off the atmosphere here it has to scatter and then come back down towards us this way so the light's coming in this way

it excites the molecules like this or like this and they're vibrating when they remit that light it's all polarized this way hence why the polarizer lets light through when it's vertical and blocks it when it's horizontal and then if we go this way which is about opposite the sun there the effects diminished and that's because the light wav is coming in unpolarized exciting the atoms in the air and they're remitting unpolarized light back at us so that's the sky is seen under polarization um some interesting facts here that bees will use this to navigate by

so light doesn't have to be just vertically polarized or horizontally polarized that's what we call linear polarization it can also be what's called circularly polarized so what that is is we had our electric field and we had our magnetic field so let's just take a look at the electric field for a minute in linear polarization electric field is going straight up and down linearly in a line in circular polarization as that electric field moves through space it's also rotating so at one point in time it's vertically polarized and another point in time it's horizontally polarized

um and just like we can have vertical and horizontal polarization we can have right circularly polarized or left circularly polarized depending on how this field rotates as it moves through space now the question is have you seen circular polarizers well I bet you have for example if you've been to a 3 movie these are real 3D glasses and how they work is a combination of circular and linear polar polarization and polarizers so on the front of these we have a circular polarizer and on the back we have a linear polarizer what that means is that

if light is incident on the front it gets circularly polarized and it'll always make it through the linear polariz polarizer well if it's unpolarized or linearly polarized light I'll demonstrate that it's not just a linear polarizer by looking at this linear polarizer through it so if we do that and we rotate it it never completely blocks out the line in fact it never really changes the intensity very much but I said there was a circular polarizer and a linear polarizer so if the light's moving this way traveling this way and encounters that linear polarizer first

it will be completely blocked so if we flip the glasses around and now rotate them ah we do see that complete blockage so again this way is linearly polarized a circular which makes it through the linear and it makes it through this way linearly polarized blocked by linear polarizer and then never sees a circular polarizer so how they do this is they send One circularly Polarized say left circularly polarized that reflects off the screen makes it through the circular polarizer and makes it through the linear analyzer on the other glasses though it's rotated the other

way and instead of say making it through horizontally and making it through this right yeah horizontally I'm making it through this um polarizer back here it becomes vertical and doesn't pass through that analyzer that second polarizer um but that was for left U polarized light we can also have right circularly polarized light which they use for the other image that one makes it through this polarizer but doesn't make it through this one that's how we can get those two different images now let's take a look and see also how these circular polarizers work see if

that's right so if we now have start with unpolarized light that first linear polarizer makes it linearly polarized and then we have the front um circular polarizer so now we have should have circularly polarized light out of the flashlight and I said that it wouldn't make it through this one and sure enough it really doesn't well there we go I have to rotate a little bit um okay so what sort of cool experiments can we do with circular polarization well one of my favorite actually involves a mirror so this might be kind of tricky for

me we're going to do is we're going to start with a flashlight on our circular polarizer so you can see some of the Lights making it through there um but now it's going to be circularly polarized going to bounce off the mirror and from this angle we can still see it from the camera turn the camera up and look through the circular polarizer oh it's extinguished most of the light what's going on there any ideas well what's happening is that after it passes through circular polarizer it is say left circularly polarized when it bounces off

of the mirror surface it becomes right circularly polarized on the way back up and that right circularly polarizer then is rotated the other way so it doesn't make it through the linear polarizer on the other side of this because remember this is a circular polarizer and a linear polarizer well thanks for watching I hope you learned something about polarization today and be on the lookout for examples of polarization in your life see you later