# Molecular chirality and optical rotation

Why does having molecular chirality result in optical rotation? The dissymetry or chirality of molecules translates to the rotation of plane polarized light, the magnitude and direction depending on the concentration and the nature of the substance. But why does molecular chirality cause the rotation of plane polarized light.
If this rotation is due to the assymetry around the bond, the different molecules are oriented in all possible directions(randomly) and hence the light will fall on them in all different orientations, and should ultimately be undeflected since for every orientation of the molecule, we can reverse the orientation such that the light appears to be falling on the molecule from a direction other than the one for our original molecule.
Moreover, why does inverting the configuration invariably reverses the direction of rotation of light?

• It seems to me that you get very close to the answer. Citing you: "since for every orientation of the molecule, we can reverse the orientation such that the light appears to be falling on the molecule from a direction other than the one for our original molecule." In order to "reverse" the orientation, you would need to have a position that is exactly the mirror image of the opposite situation. And this is exactly what cannot happen for a solution of chiral molecules.
– PLD
Commented Sep 20, 2013 at 21:31

since for every orientation of the molecule, we can reverse the orientation such that the light appears to be falling on the molecule from a direction other than the one for our original molecule.

This is false.

Let's take 2-butanol. For this stereoisomer, light is turning clockwise when viewed from the right side (I'm not sure of this, but we can assume).

Here, the disks are there to clarify the direction from which we are viewing the entire setup. Here, the "face" of the disks is to the right so we are viewing it from the right.

By the principle of reversibility of light, if light enters from the other side, we get the reverse path:

Rotate this diagram (by $180^\circ$ around an axis perpendicular to your screen):

Note that the disks are being viewed from the other side (the left) now. But we can flip them:

Take a close look at this. This is the same molecule as the first, just oriented backwards. And looking from the right, it still gives a clockwise twist to the angle.

So while rays of light incident on different orientations of a molecule may differ in the exact value of the angle of rotation, rays of light incident on a reversed (not stereoisomeric, just spatially rotated) molecule have the same direction and value of the angle of rotation and do not cancel.

[8]:

• Thanks for a great explanation. This explains a lot. On a side note, why does a molecule rotate he light which falls on it? Is it due to diffraction effects? Commented Sep 21, 2013 at 2:39
• @SatwikPasani Good questions; see physics.stackexchange.com/questions/15503/… Commented Sep 21, 2013 at 2:57