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!