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Current time:0:00Total duration:14:30

Polarization of light, linear and circular

Video transcript

let's talk about polarization of light we know what light waves are they're electromagnetic waves so they're made out of electric fields and that's not good enough we know there's not just electric fields that couldn't sustain itself there's got to be magnetic fields there as well that are changing and those are perpendicular so you can kind of draw them it's hard on something two-dimensional but you can kind of imagine those looking something like this and those magnetic fields would point at a right angle to the electric fields but this gets really messy if I try to draw both the electric and magnetic fields at the same time and so we're going to leave the magnetic fields out it's often good enough to just know the direction of the electric field and we focus on the electric field so what is polarization mean polarization refers to the fact that if this light ray was heading straight toward your eye or a detector over here what would you see well if I draw an axis over here and this point here in the middle this is this line so imagine we're looking straight down that line and then up and down is up and down and then left and right that direction I have the magnetic field would be this way in that way what would my eye see well my eyes only going to see electric fields that either point up or electric fields that point down they might have different values but I'm only going to see electric fields that point up or down because of that this light ray is polarized so polarized light is light where the electric field is only oscillating in one direction up or down that's one direction vertically or it could be polarized horizontally or it could be polarized diagonally but either way you could have this wave polarized along any direction I mean a light ray like this if we had a coming in diagonal this light ray that's oscillating like this whether you like to feel oscillates like that that's also polarized these are both polarized because there's only one direction that the electric field is oscillating in and you might think oh how could you ever have a light ray that's not polarized easy most light that you get is not polarized that is to say light that's coming Sun straight from the Sun typically not polarized light from a light bulb an old incandescent light bulb this thing's hot you can get light polarized in every direction all at once all overlapping so if we draw this case for a light bulb just a random incandescent light bulb you might get light some of the light hitting your eye you can get some light that's got that direction you got light that's got this direction you got light in all these directions at any given moment I mean you'd have to add these up to get the total and they might not be the same value but what I'm trying to say is at any given moment you don't know what direction the electric field is going to be hitting your eye at from a random source it could be in any direction so this is not polarized this this diagram represents light that is not polarized at some point the field might be pointing this way at some later point is this way it's just a random you never know which way the electric field is going to be pointing whereas these over here these are polarized so how could you polarize this light let's say you wanted light that was polarized you're doing an experiment you needed polarized light well that's easy you can use what's called a polarizer and this is a material that lets light through but it only lets light through in one orientation so you can have a polarizer that for instance only lets through vertically polarized light so this is a polarizer these are cheap then plastic configured in a way so that it only lets light through that's vertically polarized any light coming in here that's not vertically polarized gets blocked or absorbed so what this means is if you use this polarizer and held it in between your eye and this light bulb you would only get this light all the rest of it would get blocked or you can just rotate this thing and imagine a polarizer that only lets through horizontal light now it would only let through light that was this way and so you would only get this part of the light or you could just orient it at any angle you want and below everything but the certain angle that this polarizer is defined as letting light through so you can do this and once you hold this up you get polarized light light that's only got one orientation so that's what polarization means but why do we care about polarization well let me get rid of this for a minute you've heard of polarized sunglasses so imagine you're standing near water or maybe you're standing on ice or snow or something reflective there's a problem say the sun's out it's shining it's a beautiful day except there's going to be glare let's say you're looking down at something here on the ground well it's going to get light reflecting off of it from just you know lights coming in from all directions but it also gets this direct light from the Sun so gets light from reflected off the clouds and whatever you know whatever's nearby ambient light and then there's also this direct sunlight that's harsh if that reflects straight up to your eye that hurts I don't like that it blocks our vision it's hard to see it's glare we don't want this glare so what can we do well it just so happens that when light reflects off of a surface even though the light from the Sun is not polarized once it reflects it does get polarized or at least partially polarized so this surface here once this light reflects it's coming in at all all orientations you got electric field you never know what electric field you're gonna get straight from the Sun and when it reflects though you mostly get upon reflection the direction of polarization defined by the plane of the surface that it hit so because the floor is you know horizontal when this light ray hits the ground and reflects that reflected the light gets partially polarized this horizontal component of the electric field is going to be more present than the other components maybe not completely sometimes it could be it could be completely polarized but often it's just partially polarized but that's pretty cool because now you know we can do I know how to block this we just get some sunglasses we put some sunglasses on and we make our glasses so that these are polarized in which how do we want these polarized I want to get rid of the glare so what I do is I make sure my sunglasses only let through vertically polarized light here's some polarisers that way a lot of this glare gets blocked because it does not have a vertical orientation it has a horizontal orientation and then we can block it so that's one good thing that polarization does for us and understanding it we can get rid of glare also fishermen like it because if you're trying to look in the water at fish you want to see in through the water you want to see this light from the fish getting to you you don't want to see the glare off of the Sun getting to you so polarized sunglasses are useful also we can play a trick on our eye if we really wanted to you could take one of these make one I have a vertical orientation for the polarization have the other eye with a horizontal and you're thinking this is stupid what would you do this for this is going to get a lot of glare we wouldn't use these outside when you're like skiing or fishing but you could play a trick on your eyes if you went to the movies and you went and watch the movie well the reason our eyes see 3d is because they're spaced a little bit apart they Beach get a different slightly different image that makes us see in 3d we can play the same trick on our eye if we have the polarization like this if light if some of the light from the movie theater screens coming in with one polarization and the other lights coming in with the other polarization we can send two different images to our eyes at the same time if you took these off and look like garbage because you'd be getting both of these slightly different images it look all blurry and it does if you take off your 3d glasses and look at a 3d movie looks terrible because now both the eyes are getting both images but if you put your glasses back on now this eye only gets the orientation that it's supposed to get and this I only gets the or that it's supposed to get and you get a 3d image so it's useful in many ways and let me show you one more thing here let's come back to here this light was polarized vertically and so that's called linear polarization anytime same with these these are all linear polarization because just up and down one linear direction just diagonal this is also linear all of these are linear you can get circular polarized light so if we come back to here we've got our electric field pointing up like that now let's say we sent in another light ray another light Ray that also had a polarization but not in this direction let's say our other light Ray had polarization in this direction so it looks like this kind of like what our magnetic field would have looked like but this is a completely different light ray with its own polarization and its own magnetic field so we sent this in what would happen well at this point you'd have electric field that points this way at this point you'd have electric field that points that way what would your eye see if you were over here let's see if I draw our axes here alright when this point right here gets to your eye what am I going to see well going to have a light ray that's one part of the light ray one component points up that's this electric field one component points left that's this electric field so the total my total electric field would point this way I could do the Pythagorean theorem if I wanted to figure out the size of it but I just want to know the direction for now and then it gets to here and look at they both have zero this light ray has zero electric field this one has your electric field so then it'd just be a zero now what happens over here well I've got light this one points to the right at that point this pink one and then this red one would be pointing down so what would I have at that point I'd have light that one this way and it would just be doing this over and over it would just be I just have diagonally polarized light this isn't giving me anything new I think this is dumb why do this why send in two different waves to just get diagonally polarized light I could have just sent in one wave it was diagonally polarized and got the same thing the reason is if you shift this purple wave this pink wave by ninety degrees of phase by PI over two in phase something magical happens let me show you what happens here if we move this to here now we don't just get diagonally linear polarized light what we're going to get is let me get rid of this okay so we start off with red all right the red electric field points up and then this pink waves electric field is zero at that point so this is all I have my total electric field would just be up I'm going to draw it right here the green will be the total now I come over to here and at this point there's some red electric field that points up but there's some of this other electric field that points this way so I'd have a total electric field that would point that way and then I get over to here and I'd have all of the electric field from the pink one none from the red one it would point all left then look what's happening the polarization of this light if I shift this if I'm sitting here looking with my eye as my eye receives this light I'm going to see this light rotate its polarization polarization I'm going to notice swings around in the circular pattern and because of this we call this circular polarization so this is another type of polarization where the actual angle of polarization rotates smoothly as this light ray enters your eye and you know what order alright actually I said you'd receive this one first that makes no sense you're gonna receive the ones closest to you first in this light ray going this way so you'd actually receive this one first then that one then this one then this one because of that you wouldn't see this going in a counterclockwise way you'd see this going in a clockwise circularly polarised way sorry about that and you might think okay why do I even bother with circular polarization well I kind of lied earlier it turns out in the movie theater example they don't actually do it like this typically oftentimes in the movie theaters we don't have just linearly polarized sunglasses this would be a problem because when you look at the movie theater screen and if you were to tilt your head just a little bit think about it this one's not really going to get the right image anymore it's going to get some of both and this one's going to get some of both is going to be blurry your eye your head would have to be a perfectly level the whole time which might be annoying so what we do is instead we create circular polarized glasses so that this one would only get one polarization this one would get the other direction this way even if you tilt your head a little bit shoot clockwise is clockwise counterclockwise is counterclockwise by using circular polarization for 3d movies it can make it a little easier on your eyes to see a better 3d image even if your head is tilted a little bit