Question: There is probably a really simple answer to this, but this has been bugging me from chemistry lessons: if optical isomers rotate plane-polarised light either clockwise or anticlockwise, surely an 'upside-down' molecule of an isomer would rotate the light the other way, cancelling out the effect of the molecules the 'right' way up...Shouldn't this give a net result of no effect on the plane-polarised light passing through an optically active substance?

Jack Miller answered on 22 Jun 2013:

Hi Kaursing,

It’s a good question! The most basic answer your question is that you can’t switch between stereoisomers by rotation, so an upside-down chiral molecule doesn’t look like its enantiomer. However, I don’t think that’s entirely what you’re asking. I guess the most important thing to start with is to think of lots of molecules in solution: they’ll be randomly orientated, tumbling wildly in all directions as they’re hit by other things all the time. The fact that there’s any net distribution in the properties of light coming out the other end at all should tell you that what’s going on isn’t as simple as it might at first appear.

Let’s just talk about the classical description of what polarised light is first of all. Light itself is a complex phenomenon: you can view it as the mutual induction of electric (E) and magnetic (B) fields in free space, where the electric and magnetic fields oscillate in planes perpendicular to each other, and cause each-other to be generated (you can derive this picture mathematically from the set of equations that govern electromagnetism classically, called Maxwell’s equations).

Polarisation is all due to the angle these E (and therefore B) fields can take in space. Unpolarised light has a uniform distribution of angles around 360 degrees; linear, or ‘plane’ polarisation occurs when there is only one specific angle that the E field makes to some axis. Circular polarisation is like linear polarisation, but that angle itself is a function of time — it advances like a clock’s hand does.

This is the ‘classical electromagnetism’ picture, and it’s true, but it’s not complete. It turns out that a photon’s polarisation is a property that has a quantum equivalent that plays a role in the exact mechanism of how, on a very small scale, light is absorbed and later (probably on the order of fs-ns) remitted by a chiral molecule. I’m not a physical chemist, and I’ve never really studied this process in detail for complex molecules, so perhaps Fiona will be able to shed more light (pun not intended) on what exactly goes on!

Hope that helps (and, incidentally, if your molecule wasn’t free to rotate, you’d only observe light at certain angles, which can tell you things about the exact structure of what you’re looking at…)

— Jack