Molecules are optically active when they react differently to left or right circularly polarized light. This is because they have a screw-oriented structure which is itself either left-handed or right-handed, making propagation preferential for one polarisation, rather than for the other. That's all well good, and how is it that molecules become structured in this way? Is there a good reason to become structured in this way, or is it just a fluke of evolution? I say this because at the beginning of the 20th century, a text is written containing the following statement: This argument is based on a point raised earlier in the text. Apparently, at the moment the text was written, it was believed that So, I could ask a more concrete question : 2a) Is it possible to separate levo-molecules from dextro-molecules, by some operation? What is this operation? 2b) What property does such a mixture of molecules require, for such a separation to occur? In other words, what are the constituents that carry such a preferential handedness? What is the property of these constituents related to such handedness? 2c) Can every molecule (or every constituent in a molecule) somehow be adapted to alter this property, thereby changing its handness? What is an operation that can make this happen? More questions in this line of thought could be asked, as I'm sure you can guess. So what is to be said about designing optical activity?