# Can a compound optically active in visible light also show optical activity in radio waves region?

Optically active compounds such as glucose rotate the plane of polarization of light because they lack symmetry. As I was reading up on the underlying principle behind this effect, at its core, this effect seems to have its basis in EM field theory in the way that left and right circularly polarized light have different propagation velocities in such compounds. If that's the case, shouldn't all electromagnetic waves be affected? Almost all the articles I have read, deal with optical activity in visible light. Will long wavelength waves like radio waves also have their plane of polarization rotated? Or are their wavelengths too long to prevent any interaction with molecules, if at all this is the case?

In fact this kind of effect can theoretically happen over the whole range of the EM spectrum. As you describe correctly, the source of the effect comes from the different propagation velocities for the two different circular polarizations. If you take for example a sugar solution and visible light, you will be able to observe the effect. When extending the experiment to other light wavelengths you basically have to look at the dispersion relation of the two circular polarizations. If you now take the difference between the two polarizations you can define something like an optical rotation dispersion (ORD). So your question can be reformulated into "How does the optical rotation dispersion of some material looks like?" The green curve in the image (taken from here) tells you this for an organic compound.

So as you see, the optical rotation goes zero when the wavelength increases. The reason for this behavior is that "your wavelength is becoming too big to see the chirality of the material". So with the size of the chiral objects comes a size scale in which the effect can be observed. For near/far infrared, this kind of effect and much more complicated ones are well known and used for all kinds of applications (filtering, refection, superlenses, negative index materials,...). One buzzword to look for is metamaterial. For IR light the chiral structures are in the low µm regime. For the radio frequency it goes to the mm regime.

To answer your initial question: No, visible light optically active materials are not active at radio (<1THz) frequencies, as the wavelengths are too big to "see" the chirality of the material.

Optical optical phased array for µm wavelengths. From here

Negative index material from here.

• Great answer, but I think it probably should conclude with an answer to the header question, which would be, no, visible light optically active materials are not active at radio ($<1 THz$) frequencies where the wavelengths are too big to "see" the chirality (to use your wonderfully evocative phrase). – Selene Routley Jan 6 '17 at 11:59
• Good point @WetSavannaAnimalakaRodVance I updated the answer and used your wording - thanks. – user_na Jan 6 '17 at 12:31