13.9 Polarization

Okay, today's lesson is on polarization, and this will wrap up our chapter on polarizing. There are no formulas or anything to calculate; it's mostly theory to help us understand a kind of weird thing that we've all witnessed but is pretty hard to explain in some cases. Over the years, polarization has been observed in various situations. For example, we experience glare when light reflects off water or snow, or when sunlight reflects off glass or other surfaces. The reason we get glare is due to polarization. So, what is polarization? First of all, we need to think of a wave as being three-dimensional. It expands in every direction. If we were to look at the end of a wave traveling towards us, it would look something like this picture. We have an equal wave going in every direction, vibrating in 360 degrees. What happens next is that once that wave... ...It's surface, like water, part of it gets crunched so you can see in this picture. If just to keep it simple, we got the wave is just being like an X Y axis. So you can see once that wave would travel down and hit the water, the X, though the vertical part, would actually get crunched. So think of it like a basketball. When a basketball bounces and hits the floor, it kind of compresses, so the vertical part gets crunched and the horizontal stretches out a little bit. So you can see what's gonna happen then is that wave gets crunched vertically, stretches out, and that's what gets reflected off. The idea with polarizing is there's nothing weird going on, the problem is our eyes and brains. We can't figure out what light waves like that look like. So anytime a light wave that's not proportional, so we normally should always have a wave that's sort of equal in height and width, if it gets distorted. In any way, our brain doesn't know what to do with it, and we just sort of see it as glare. So, to get rid of the glare, we have to try to get that wave back to normal. So, if you think of the water wave as being kind of like that, after this water, the ideas we just need to make it both the same size. So, if we can get the width to match the height, the wave will then be back to being proportional and we'd be able to see it. We lose some of the brightness and some of the features of our but we'd still be able to see it. So, what happens is we need to balance it up, so you got to think of like polarizing lenses sort of like being a picket fence. If you have a fence and we had a skipping-rope grow through the fence, we could create the wave vertically up and down through the fence boards, but if we tried to go sideways, they would get stopped, the boards would stop the rope and our wave wouldn't go through very good. So, what a polarizing filter does then is it just sort of makes those waves back to normal. Can see in this picture again, we got our normal sunlight wave hitting the water. It gets crunched down, so once it goes through a polarizing filter, if we have a vertical filter in this case, the vertical part of the wave would go through it, just like a rope would go through a fence board. But the horizontal part would get chopped off depending on the width of the cracks. So in this case, what's gonna happen is, we would come out the same height but our width would get shortened down. Now, if they're the same size vertically and horizontally, then we'd actually be able to see the picture. So polarizing lenses, that's all they do, is try to make the wave the same. So quite often, sunglasses, polarizing sunglasses, the idea is just to do the exactly this. You have vertical lines cut in it, and it's to help cut out the glare off of anything that's shiny. Because of that, knowing how to use polarizing lenses can actually work pretty well for photographers. So you can... See, in this picture, there's a woman talking on the phone and we're taking a picture through the window. So, in this first picture, they didn't clear off the window and you can't really see much. If we put a filter on it, we would be able to see better. The filter on the camera is just a bunch of vertical lines that top off the glare coming through. It doesn't necessarily have to be vertical, it could be horizontal depending on the direction of the reflection. You might have to turn the lens one way or the other to cut off the glare. If we think of a polarizing filter as a screen of vertical or horizontal cuts, when we rotate them, we can achieve different effects. For example, if we try to pass light through a vertical one only, the waves pass through fine. However, if we try to pass it through a horizontal one, they don't. So, if we use a combination of a vertical and a horizontal filter, it would basically... Eliminate almost all of the wave, so nothing would pass through. So, if you take polarizing lenses (the easiest example would be 3D movie glasses), you can see that they have one lens vertical and one lens horizontal. The light gets blocked out because when you rotate the second pair of glasses by 90 degrees, you're essentially blocking out all of the vertical and horizontal light at the same time. This cancels out everything. So, how do 3D glasses work? Well, 3D movies are essentially filmed twice. When they're recording movies and the actors, what they do is they have one camera. And what I did here, I lost my picture, but that's okay. We don't need it anymore anyway. So what they would do then with the movie is they film one camera that shines, films the movie, and it has a vertical lens on the camera. The other camera films the movie with the horizontal lens. And what they do, anything that they want to have looking 3D, what they do is they film that scene twice. And they just get the actor or whatever they want to be 3D in a different location. So that's opposed, you know. They have a person here. Both cameras film that person at the same time. But then they do a second take and they move. Let's suppose. They move the person's fist further outwards, and they only fill it with this camera. The other camera simply keeps the original image. So what happens is, the person's fist will kind of overlap. That's why if you look at a 3D movie without 3D glasses on, it looks kind of blurry. Because anything they want to show in 3D is filmed a little bit differently than the original. Then the two images sort of kind of overlap each other, and it's not quite aligned. So you get this little bit of a blurry effect. Now, when you're watching the movie with 3D glasses on, one of the lenses is filmed horizontally, and the other one has vertical polarization. Each eye sees what each of the cameras recorded. So if they have a 3D effect where one lens is showing it a little bit differently, you'll see that in one eye. But then you... Can you see the original image in the second eye? It's just a brain trick. After that, your brain doesn't know what to do with two different images that aren't quite exactly the same. So what your brain thinks is, "Well, it's moving forward or it's a little bit further out," so it thinks it's more of a 3D image, like what you'd see in real life compared to just a 2D flat movie. Take the image, and that's all.