Re: How do optically active compounds rotate plane polarized light?
From: Repeating Rifle (salmonegg_at_sbcglobal.net)
Date: 02/04/05
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Date: Fri, 04 Feb 2005 21:23:50 GMT
in article 1107521787.991417.61100@z14g2000cwz.googlegroups.com,
farooq_w@hotmail.com at farooq_w@hotmail.com wrote on 2/4/05 4:56 AM:
> A simple question whose answer I am unable to find in the undergraduate
> level organic chem. texts I have checked (Morrison, Solomon etc.). Why
> and how do the optically active compounds rotate plane polarized light?
> A preferrably online pointer to a reference providing non-mathematical
> discussion of the reason behind it would be most helpful.
>
> Secondly, are there any known examples of optically active compounds
> which do not contain carbon and hydrogen at all?
>
If you look at a book such as "Principles of Optics" by Born and Wolf. There
is a good section on crystal optics. Unfortunately, even Born and Wolf does
not get into circular birefringence. It does not treat how the chiral
structure is related to the dielectric tensor.
The most common example of an optically active crystal without carbon or
hydrogen is that of quartz. The crystals come in left and right hand forms.
On a macroscopic scale, when a polarized wave enters a crystal, it breaks up
into waves. The most usual situation is for a uniaxial crystal where it
breaks up into ordinary and extraordinary waves with different velocities of
propagation. Each wave propagates indepentently without polarization change.
Leaving the crystal, these waves are sometimes recombined to give a
polarization state for the exiting wave that is different from either one of
the propagating wave. This is how retarders work. These waves are sometimes
considered to be eigenvectors or eigen states of light.
While the eigenvectors (ordinary and extraordinary waves) are linearly
polarized in the simple cases, they may be circularlly or elliptically
polarized. For example, right and left hand polarizations are the
eigenstates for waves traveling down the optic (rotational symmetry) axis of
quartz. They travel at different velocities. The result is that linearly
polarized light breaks up in right an left forms that recombine to give
linear polarization again albeit rotated with respect to original
orientation.
I know this is complicated, but there is no way that I know to simplify it
more. I think there is an article in the Handbook of Physics/Handbuch der
Physik. I may also have a translation of a French treatise over a hundred
years old in my archives somewhere.
Bill
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