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Polarization

by Jared Rovny
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    00:01 If we look at this light wave that we just talked about head-on.

    00:04 So suppose this light wave is heading right out of the page right at us.

    00:07 We could see the electric field component and the magnetic field component separately.

    00:12 And we would see as we were watching this wave, both of them oscillating as it moves towards us.

    00:18 But for now let?s just focus on the electric field component.

    00:21 So just looking at this electric field as it oscillates.

    00:24 We would see that it would go up and down and up and down as the wave oscillated while the photon, which we'll discuss later, this electromagnetic wave moved.

    00:34 This is, in this particular picture what we would call linearly polarized light because by that we mean that the electric field is just moving in a line.

    00:45 It is just moving up and down.

    00:46 It's not moving side to side and there are no other electric field components to this particular image of light.

    00:52 So this is linearly polarized light whereas we couldn?t, principle have many different components or many polarizations if you will.

    00:59 Many different directions that the electric field is oscillating in because we can have a more complicated electromagnetic wave.

    01:07 We could have other kinds of polarizations though.

    01:10 The total polarization for a light as a wave would be the sum of all of those different components that I've just showed.

    01:17 So if we have electric fields pointing in different directions as the light propagates forward the actual polarization of that light.

    01:24 In other words, the net sum the vector sum of the different electric field components, would just be again the sum of these.

    01:30 This gives us as something interesting we could do.

    01:33 Suppose that we have just two components, not many but not one where it's linearly polarized but just two components.

    01:39 One of which was perpendicular to the other one.

    01:42 And we could ask ourselves if these two components of the electric field were slightly out of phase with each other.

    01:49 In other words, instead of oscillating in the same way where they're both in time with each other in track.

    01:54 What if they're oscillating at a slightly different rate as each other? They're sort of out of phase. Let?s watch what happens if we do this.

    02:01 We have two components to our polarization, suppose we had a vertical one which I'm going to draw in green here.

    02:07 And we would also have a horizontal one which I will draw in blue but because these are out phase we're going to assume that initially the blue component, the horizontal component is at its zero point.

    02:18 It's crossing zero right now.

    02:19 The vertical component, the green component right now is at its maximum value.

    02:24 What we're going to do is like I said track the total electric field component by adding the green and the blue vectors.

    02:31 And we'll track that with the red dot here which will be our total electric field as a vector the position of it.

    02:38 So let?s watch, let?s let the green field slowly decrease as it's oscillating down and then up.

    02:44 It's on its way down right now.

    02:45 The blue field as we said is crossing the zero point, so it just crossed zero and it's increasing.

    02:50 Notice what happened to the red dot which is the total of the two, the green and the blue.

    02:55 The total electric field is this red line ending in the red dot which is moved slightly.

    03:02 It's rotated a little bit around this red circle and we could keep this going.

    03:05 The blue has now completed its path towards the maximum.

    03:09 The green, the vertical component of the electric field, has now crossed zero and now the red dot is on the side.

    03:15 So it's gone a full quarter turn. If we keep doing this, this will continue.

    03:20 The red, sorry the blue will keep oscillating horizontally.

    03:23 The green will keep oscillating vertically and the vector sum or the total electric field from these two components will rotate as these two oscillate in their independent directions.

    03:33 We call this circularly polarized light because as the light wave moves forward the total electric field component is moving in a circum-, as we just discussed.

    03:44 It turns out that for metals, we have the electrons in metals and we already know that metals are good conductors.

    03:51 Meaning that the electrons are free to move if an electric field or some sort of force is applied to the electrons in the metal.

    03:59 This means that if electromagnetic waves are moving towards the metal, the electrons in the metal will see that electric field and just as we discussed when we were talking about electromagnetism, will respond to the electric field.

    04:12 They will feel a force and they'll start moving up and down.

    04:14 What this means is that this metal is a good absorber of the energy from an electromagnetic wave. In other words, the metal can absorb the energy from light and also reflect it back because the electrons can move and response.

    04:30 This is in fact what makes metals so shiny, when light impends on the surface, what happens is the electrons on that surface begin to move in response to the electric field component of the light and then just like a rope, if we were attached to something, will be reflected off that surface if that surface can move especially and the light would bounce back off and we have a very reflective surface.

    04:52 Something more common to use this effect would be to polarize light.

    04:56 So if we have an electric field component which is in many different directions for this incoming electromagnetic wave.

    05:03 If we pass it through a filter like this one which has some sort of a conductive material vertically in this case with little openings in the material.

    05:11 The component of your electric field could be absorbed by the motion of the electrons in the metal leaving you with just one component to your electric field.

    05:19 So we can polarize the light in this way.

    05:21 So you can see what comes out of the polarizer is a polarized direction of your electric field with the rays still moving in the original direction.

    05:29 What we have here is something that?s vertically polarized but you could also do this in different directions, maybe horizontally polarized.

    05:36 And in fact this is used in for example your sunglasses, if you have polarized sunglasses.

    05:41 The idea is that when light reflects off of a surface like the road in front of you it bounces off that surface and comes to your eyes.

    05:48 It is already horizontally polarized because the surface can also act like a conductor in the same way that we just described.

    05:55 And so the horizontally polarized light heading towards your eyes could be matched with a vertically polarized filter from your sunglasses blocking the glare that might otherwise come to your eyes.

    06:05 And so we could use this effect in many practical circumstances.

    06:07 So this completes us some of our further properties of light and how we treat light both as an electromagnetic wave.

    06:14 As well as a way which has an electric component which can rotate or be linearly polarized or circularly polarized.

    06:21 So we have one more discussion of light to come and until then thanks for listening.


    About the Lecture

    The lecture Polarization by Jared Rovny is from the course Light: Electromagnetic Radiation.


    Included Quiz Questions

    1. The combination of components of the electric field
    2. The sum of electric and magnetic field components
    3. The direction of the energy of the light
    4. The direction of the magnetic field
    5. The direction of motion of the light
    1. Perpendicular components of the electric field, out of phase by exactly 90 degrees
    2. Parallel components of the electric field, out of phase by exactly 90 degrees
    3. Two wavelengths of light in a ratio of exactly 90 degrees
    4. Parallel magnetic field components oscillating together
    5. The spinning rotation of photons as they move
    1. By sending the light through a medium which absorbs all the components of the electric field but one
    2. By sending the light through glass
    3. By sending the light through a medium like water
    4. By emitting very low frequency light
    5. By shifting the phase of the light through a thin film
    1. By using a vertically polarized filter
    2. By using a horizontally polarized filter
    3. It will be blocked by the glass
    4. By using any thin glass
    5. Using a circularly polarized filter

    Author of lecture Polarization

     Jared Rovny

    Jared Rovny


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