Take a look at Fresnel coefficients. Stop and take a look at what you know about Fresnel coefficients and how the reflectance of a surface depends on the polarization of light. What we're going to do here is we're going to take a basic piece of plastic, we are going to send in a single ray of light, which we can use white light in this case. We're going to look at the reflection from the front surface. So, for this case I have polarized the beam. I've put a linear polarizer in front of our white light source. So, we have either light that is polarized parallel to the table, so the electric field is oscillating parallel to the table, or the light vector has electric field that's oscillating perpendicular to the table in and out. So let's look at the reflected light. You see here I have a little bit of reflected light coming off at about normal incidence, and as I increase the angle of rotation, what is happening to the intensity of that light? Now, we're starting to see a little bit more here and now it's starting to get quite bright. So, now, as I get closer and closer to a 90 degree incident angle, I get a much larger percentage of my light that's reflected, then is transmitted, and so I get a very very large reflected signal at normal incidence, an incidence angle of about zero degrees, we got a pretty small reflected angle. So, using what you know about the Fresnel coefficient, what is the polarization of this light? We told you it's either parallel polarized or perpendicular polarized. So, which one is it? So, if you look at the Fresnel coefficient and you plot them out versus angle, the reflected intensity for the perpendicular polarized light, the light that's perpendicular where the electric fields oscillating in and out of the table, has a profile which starts out pretty small and eventually increases towards 90 degrees. So, let's look at what the other polarization does. I'm going to take out the polarizer here. Intensity goes up just a little bit. I'm going to put it back in to the other orientation. So, now, I'm going to have only light that is polarized parallel to the table incident on this piece of glass. So, here you see, I have a reflected beam that is once again quite pale. A little bit difficult to see, not very much light is reflected. As I rotate this I start to see much of anything in. I'm rotating it around and it looks like that reflected beam has completely disappeared. I'm going to keep rotating and now it starts to reappear. As I get close to an incident angle of 90 degrees, then I will get a pretty bright reflected beam again. So, it's a little bit hard for me to have an incident angle of 90 degrees here. As I rotate that back down, it gets smaller and smaller, and it's a point actually goes to about zero, a little tinnier somewhere. A little bit hard to see the beam perfectly, and this is called Brewster's angle. This is the angle where light that is polarized parallel to the table in this case will not be reflected at all. It will all be transmitted. If I now take the same angle and don't move the glass, it would change the polarization of the light to perpendicularly polarized. I now get a reflected beam. So, this will only happen, this reflection going zero only happen for the light that is parallel polarized. As I continue to rotate, we start to get a reflected beam again and it's pretty small, but you have a reflection down at smaller incident angles. So, what happens when I removed the polarizer altogether? Well, first of all my intensity goes up a little bit. Any polarizer in front of unpolarized light is going to drop the light intensity. So, now I have a beam of light that's hopefully it'll be easier to see. I see if I look at the reflected beam as I increase this, what's going to happen at Brewster angle? Is the light reflected going go to zero or is it not going to go to zero? Let's see. So, I increase, I don't remember exactly where it was but it was probably in here. It does not look like the reflection goes to zero. Why is that? Well, this is now a combination of light that's polarized parallel and light that's polarized perpendicular. So, light that's polarized parallel goes to zero but there's still lots of light that is polarized perpendicular that is reflecting off of the sample.