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Optical activity

  1. Jan 20, 2014 #1
    "Optical activity is the tendency of chiral molecules to rotate ppl" how does a chiral molecule rotate light? Is it because of interaction of electric fields? Anyone please help me? I just need to know how a molecule physically does that?
  2. jcsd
  3. Jan 20, 2014 #2


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    The electric fields of the optical wave polarize the electron clouds. The moving polarization of the electron clouds in turn leads to electrical fields which are partially emitted. However, the electric fields are not always emitted at the same point where the polarization and currents have been induced. If the resultant fields are furthermore rotated with respect to the incoming wave due to the low symmetry of the molecule, the polarization axis of the resultant field will be rotated.
  4. Jan 21, 2014 #3
    I struggle with this concept myself too. In solution, molecules are rotated in every direction so you'd think that the polarisation induced by the electric field of the wave would be isotropic and cancel each other out. But I suppose only some of the orientations (meaning which way they're rotated) can rotate the light significantly, so the other orientations can be ignored.

    I don't fully understand your explanation DrDu. When the incoming wave polarises the electron cloud, how does the emitted electrical field cause the incoming electrical field to rotate? Polarisation of electron clouds, thats just a shift of electron density isn't it? Is that what you mean by polarisation, or are you saying the electron waves get polarised in the same way light waves do? I think there should be a different term for EM polarisation, its more to do with anisotropy than polarity.
  5. Jan 21, 2014 #4

    How does the electron cloud gets polarized? And even when it gets polarized due to the lights electric field how does it help to rotate the plane of ppl?
  6. Jan 21, 2014 #5


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    Electric fields exert forces on the electrons and oppositely on the nuclei. Hence the centers of charge will separate and you will get a polarisation varying with the frequency of the incoming light. However, if the molecule isn't symmetrical, the polarisation will not be strictly along the direction of the incoming field. Hence the field of the fluctuating dipoles will be tilted as compared to the incoming field, i.e. the plane of polarisation gets rotated.
  7. Jan 21, 2014 #6
    Thanks a lot DrDu :)
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