Optical Activity: How Do Chiral Molecules Rotate Light?

In summary, a chiral molecule rotates light because of the electric fields that are emitted. The fields rotate the polarization of the electron clouds, which in turn rotates the plane of polarization.
  • #1
Vivek des
9
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"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?
 
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  • #2
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.
 
  • #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, that's 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.
 
  • #4
DrDu said:
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.


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?
 
  • #5
Vivek des said:
How does the electron cloud gets polarized?
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.
 
  • #6
Thanks a lot DrDu :)
 

1. What is optical activity?

Optical activity is a phenomenon in which chiral molecules, also known as molecules with a non-superimposable mirror image, rotate the plane of polarized light passing through them.

2. How do chiral molecules rotate light?

Chiral molecules rotate light because they have an uneven distribution of electrons, which causes them to interact differently with polarized light. This interaction results in the rotation of the plane of polarized light.

3. What is the relationship between a molecule's chirality and its ability to rotate light?

The relationship between a molecule's chirality and its ability to rotate light is that only chiral molecules have the ability to rotate light. Molecules with no chirality, such as symmetrical molecules, will not rotate light.

4. How is optical activity measured?

Optical activity is measured using a polarimeter, which is an instrument that measures the rotation of polarized light passing through a sample of a chiral molecule. The amount of rotation is then used to determine the concentration and purity of the sample.

5. What are some real-life applications of optical activity?

Optical activity has many real-life applications, including in pharmaceuticals, where it is used to determine the purity and concentration of chiral drugs. It is also used in the food industry to detect additives and in organic chemistry to determine the structure of chiral molecules.

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