The Phenomena of Optical Activity. Terms and Definitions. Theoretical Considerations. The Drude and Moffitt Equations

Abstract

When a ray of monochromatic polarized light strikes a solution, several events may occur: (1) reflection on the surface, (2) refraction, (3) rotation of the plane of polarization, and (4) absorption. Reflection can be minimized by proper optical arrangement and will not be discussed here. However, the other three events are important in the problems to be considered. Refraction is caused by slowing of the ray. In the language of the electromagnetic theory of light and electronic structure of matter, refraction is caused by interaction of the propagated electromagnetic field with the oscillating electrons of matter. A propagated electromagnetic field, according to the classical theory of Maxwell, is characterized by an electric vector E and a perpendicular magnetic vector H. If the light is monochromatic, i.e. has a definite wavelength λ or frequency v (λ=1/v), the electric vector E at any point is expressed as E=E₀, cos w t, where E ₀, is the maximum amplitude of the wave, ω = 2πv, and t is time. A similar expression, H=H_o cos ω t, is valid for the magnetic vector H, which shows that E and H, though perpendicular to one another, are in phase. In a refracting medium the wave is slowed and the amplitude of the electric field component E is diminished, and, according to the theory, the decrease is proportional to the factor 3/(n² + 2), where n is the refractive index. The decrease in the magnitude of E is caused by induction of dipole moments in the refracting molecules, and the disturbance depends on the number and distribution of the electrons. In a nonabsorbing isotropic medium, e.g. a solution containing only symmetric molecules, refraction will be the only major event. All secondary waves emerging from the solution will be polarized in the same direction as the incident wave, i.e. no optical activity will be observed. In order to generate optical activity, the substance interacting with the light should be able to affect not only the electric but also the magnetic vector (see e.g. Schellman, 1958).