What is occuring on the quantum level when a molecule rotates plane polarized light?

Also, why do enantiomers then rotate light in opposite directions? I would think that the electromagnetic waves would interact with the transient electric and magnetic fields within the molecule itself as it propagates and therefore the light changes direction as it passes through the molecule. However with this idea there seems to be a problem with the fact that all the molecules wouldn't be oriented in the same directions and thus scatter the light in all directions rather than a net rotation in one direction of the light. I am looking for an explanation using electromagnetism if it can be explained at that level or if it cannot, the quantum mechanical answer would be fine as well.

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    $\begingroup$ Possible duplicate of Molecular chirality and optical rotation $\endgroup$
    – Mithoron
    Commented Oct 11, 2017 at 12:39
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    $\begingroup$ This definitely isn't a duplicate. $\endgroup$ Commented Oct 11, 2017 at 12:40
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    $\begingroup$ I agree with @pentavalentcarbon. The post Mithoron linked is related to part of your question (why does the random orientation of a solution not cancel out the net rotation), but it doesn't address the physical mechanism by which optical rotation occurs. $\endgroup$
    – Tyberius
    Commented Oct 11, 2017 at 14:00
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    $\begingroup$ I'm not sure if it counts as a duplicate if it's on a different SE, but see how do optically active compounds rotate plane polarized light. $\endgroup$ Commented Oct 12, 2017 at 3:24
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    $\begingroup$ @a-cyclohexane-molecule A version of that answer in terms a chemist could understand would make a decent answer here (especially if you can avoid any quantum mechanics or math). $\endgroup$
    – matt_black
    Commented Oct 12, 2017 at 11:23

1 Answer 1


As mentioned in the comments, the answer to your $2^{nd}$ question regarding why optical rotation isn't simply cancelled out in solution due to random orientations is given in ManishEarth's answer to Molecular chirality and optical rotation

For your first question, I will try to explain what actually causes rotation when light impinges on an optically active molecule. For chiral molecules, it simplifies the picture to think of each enantiomer as a screw of negative charge that is either right or left handed. Plane or linearly polarized light can always be decomposed into circularly polarized clockwise and counterclockwise components. When the light collides with the molecule, the clockwise component and counterclockwise components interact differently with the electric field of the molecule, leading one of these components to travel faster than the other, which alters the angle of the plane polarized light. A good visualization of this effect (for a whole solution rather than an individual molecule) can be found at http://cddemo.szialab.org/.

As an interesting aside, you might imagine that because we have a reasonable understanding of why optical rotation occurs, we should be able to predict, either heuristically or from a first principles calculation, the magnitude and direction of optical rotation for a molecule just based on its structure. This, thus far, has not been the case. Not only are there very few ways of heuristically guessing the magnitude/direction, but quantum mechanical calculations at high levels of theory have proven unable to even consistently obtain the correct direction of optical rotation. Given a particular chiral molecule, we still can't predict which direction, let alone magnitude, that a particular enantiomer will rotate light.

My main point in this little tangent is that even something as seemingly simple and in such widespread use as optical rotation is still not very well understood and is a difficult area of active research.


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