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The question is: How can I get a sensible and concrete (not relative) value for the number of molecules light has interacted with to cause an optical activity phenomenon?

Specifically, my issue in answering the question is that although initially I thought of calculating the number of molecules contained in the volume that the light is occupying, this does not seem correct to me. The reason is that the optical activity is independent of the volume, it only depends on the path length but not the transverse area. So it seems to come down to some kind of "effective" number of interactions, which is invariant to the transverse spread of the light.

Thanks for any help you can give me!

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enter image description here

I think the picture above is self-explaining.

Now you must know the diameter and lenght of the lightpath so you can calculate the illuminated volume. Taking this volume and the concentration c you get the mol quantity (6.022 * 10^23) of molecules which have caused optical activity.

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    $\begingroup$ Can you please type out whatever text there is in the image and quote the source? $\endgroup$ – orthocresol Nov 9 '15 at 17:19
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    $\begingroup$ Thank you for writing this. My issue is exactly concerning such a volume calculation, as explained in the question: It does not make sense to me since the optical activity does not depend on the diameter of the light. However, maybe the concern was a bit hidden before. I have now shortened the question to make it more apparent. $\endgroup$ – Daria Nov 10 '15 at 12:28

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