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Conceptual problem on Polarization

  1. Mar 25, 2007 #1
    I am a beginner in polarization, and questions keep raising when I revise this topic:frown: :

    Scattering:
    1. After the molecule absorb the energy of incident light wave, the energy is reemitted in all direction. Why is it still a kind of polarization if the light emitted is in all directions? Are the molecules vibrate in all direction after absorbing light energy?

    2. If we look at the molecule at certain angle, it is said that the scattered light is partially polarized in the horizontal direction. Why does the intensity of the scattered ray in the vertical direction become weaker? I think that the molecule not just vibrate vertically but also vibrate in the direction prependicular to the ray.

    3. Why is a drop of milk needed for scattering?

    Polarization of microwaves:

    1. When the microwave pass through the metal grid which is placed vertically, it is said that the free electrons are forced to vibrate in the metal rod because of the electric field of microwave, and then the energy is absorbed by the electrons and no microwave can pass through the metal grid. Why doesn't the energy absorbed reemitted just like scattering?

    Amplitude after polarization:

    1. Sometimes the polaroid is placed with a certain angle to the electric field of the incident light, and after passing through the polaroid, the amplitude and the intensity become smaller, the direction of the electric field also change. So does that mean there is any energy loss?

    2. In the textbook, it says that if the polaroid is placed in a direction prependicular to the electric field of the incident light, the free electrons do not have space to vibrate vertically along the electric field. So how about if the polaroid is placed at a certain angle? I think that there is also no space for vertical vibration of the electrons.

    I am really confused in this chapter and I believe that I have a lot of misconception. Thank you so much for answering my questions!!!o:)

    (Also thank you for answering my last post about circular motion^^)
     
  2. jcsd
  3. Mar 25, 2007 #2
    When the microwave pass through the metal grid which is placed vertically, it is said that the free electrons are forced to vibrate in the metal rod because of the electric field of microwave, and then the energy is absorbed by the electrons and no microwave can pass through the metal grid. Why doesn't the energy absorbed reemitted just like scattering?

    Hi,
    When an electron receives an EM wave, it oscillates in the direction of the electrical field of the wave. So doing, it radiates an electromagnetic gave polarized in the direction of oscillation in all directions (but with a maximum perpendicular to movement and a zero in the direction of movement). If the electron is in a metal with a lot of others electrons, and if the metal is a good conductor at these frequencies, the phase of the emitted electromagnetic wave is such that added with the incoming one, gives a zero for the electric field at the metal level. This condition imply than the wave emitted in the same direction as the incoming wave, added with the incoming wave, annulate. The wave emitted in the direction opposite to the incoming wave has the same amplitude and polarization as the incoming wave and is seen as the reflection of the incoming wave.

    This is possible if the metal conducts in the direction of the electric field of the EM wave. In this case the grid seems to reflect the wave. If the wires are perpendicular to the electric field, there is not an emitted wave and the grid seems to let the incoming wave pass thru.

    In any case if the metal is a good conductor, there is not absorption of energy.

    For the other questions in your post, I will see latter.
     
  4. Mar 25, 2007 #3
    Polaroid polarizers:

    A polaroid filter absorbs the light polarized in a direction and lets the other direction pass "without" absorption (well just a little).

    When the light is polarized at an angle you can decompose it in two components polarized at 90°. You chose one of the directions as the absorbed one, and the other passes. Then the light emerging a polarizer is polarized in the "good" direction.

    To aggravate your problems, I can give you a surprising experiment: You look through two polaroids crossed: no light passes. Then you put another polaroid between the first two, at an angle: some of the light passes.

    Explanation: Imagine that light exits the first with vertical polarization. If the in between polarizer is at, say 30°, the component of the light with the electrical field in this direction passes. Then the component at 60° of this component passes the second. The final amplitude of light will be multiplied by cos(30°)*cos(60°).

    The big difference between the grid in microwaves and the polaroid is that the grid do not absorbs energy but the polaroid do.
     
  5. Mar 26, 2007 #4

    3. Why is a drop of milk needed for scattering?


    You do not need milk drops for light scattering. Look at the blue sky: the light you see is scattered by air molecules. Overcast? No problem. Looking at the clouds you see the light scattered by droplets of water or tiny ice crystals.

    But as water ice and droplets scatter light better than just air, droplets of milk dispersed in water scatter light better than water molecules alone.
     
  6. Mar 26, 2007 #5
    Scattering:
    1. After the molecule absorb the energy of incident light wave, the energy is reemitted in all direction. Why is it still a kind of polarization if the light emitted is in all directions? Are the molecules vibrate in all direction after absorbing light energy?


    If a molecule really absorbs the light, when it emits the energy absorbed, the light is no more polarized. This is the case in fluorescence and phosphorescence.

    In scattering the light is no absorbed. The electrons in the molecule are put in motion by the electrical field of the electromagnetic radiation and absorb energy form the wave. This motion has the direction of the polarization of the incoming wave. Electrons in motion radiate this energy as another electromagnetic wave at the same frequency and this emitted wave is polarized in the direction of the motion of the electrons. This emission is maximal in the plane perpendicular to the direction of motion and zero in the direction of motion. This explains the partial polarization of the blue light of the sky.
     
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