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        Matter acts on light. This action leads to a number of physical phenomena, such as refraction, diffraction, absorption, etc. These phenomena may depend on the polarization of light and we shall use the general term "optical activity" to describe the ability of matter to discriminate between different kinds of polarized light. The first form of optical activity (birefringence) was discovered by Bartholinus in Iceland Crystal in 1669 and qualitatively explained by Huygens’ using the wave concept of light a few years later (See Short History of Optics). However, under the influence of Newton’s corpuscular theory of light, little attention was paid to the phenomenon until the end of the 18^th century when a rapid sequence of discoveries, notably by Young and Fresnel, established the wave theory of light. Light was recognized to be a transversal wave, which can exist in various forms of polarization (Fresnel, 1822). The nature of the transversal light wave, however, remained obscure. The breakthrough came in 1849 when Faraday discovered that the polarization plane of light running through matter is rotated under the influence of a magnetic field (Faraday Effect). The discovery suggested light to be an electro-magnetic phenomenon, a view that was corroborated by Maxwell’s solutions of his celebrated equations for the electro-magnetic field in free space. The transversal "Maxwell waves" were unambiguously identified with the "Herzian" waves (= radio waves) by Herz in the period 1885-9. The Faraday effect is only found for light passing through matter. Apparently, the magnetic field changes the state of matter such that it acts differently on light with left and right circular polarization. Once the electro-magnetic nature of light was established, microscopic theories for the action of light on matter were developed by Maxwell, Helmholz, and others. Their models considered matter as an ensemble of elastically bound charged particles and were applied to explaining various aspects of refractive dispersion. The discovery of the magnetic splitting of the sodium D lines by Zeeman (1897) provided insight into the effect of a magnetic field on the microscopic structure of matter. The theoretical analysis of the periodic motion of electrons in a magnetic field, given by Lorentz in conjunction with Zeeman’s experiments, gave a theoretical basis for magneto optical effects (Faraday Effect, Magnetic Circular Dichroism, Magnetic Linear Dichroism) in the context of classical physics. Simultaneously to the elucidation of light’s electro-magnetic nature and its interaction with matter, a number of new forms of optical activity where discovered, notably the phenomena of natural optical rotation by Arago (1812) and dichroism by Biot (1815). Pasteur’s discovery of enantiomers in studies of the rotatory power of tartrate emphasized the importance of the 3-dimensional structure of matter and paved the way for the development of stereo chemistry. Despite its early discovery (37 years prior to the Faraday Effect) natural optical rotation has proved to be the most controversial form of optical activity (See Mechanisms for Natural Circular Birefringence).

        The short history presented above reveals that the major discoveries in the field of optical activity were made in the 19^th century and important insights in these effects were obtained prior to the event of quantum mechanics. The goal of this homepage is to give a non-expert reader the opportunity to understand the basic physics behind the various forms of optical activity. For these reasons, we have chosen for a presentation based on the principles of classical physics. Moreover, we believe that the classical theory is a good starting point for understanding the quantum-mechanical treatments of the subject matter.

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