Polarimetry

Polarimetry

Some substances are optically active, i.e. they bend the plane of polarized light that passes through them. From a chemical point of view, these substances are characterized by the presence of a so-called chiral center - in the case of biochemically important molecules, it is a carbon atom to which four different substituents are attached (ie the substance is asymmetric).

It is difficult to explain why non-symmetric substances bend the plane of polarized light without a deeper physical explanation. So let's content ourselves with stating that plane-polarized light is created by the composition of two circularly polarized radiations, which differ only in the direction of rotation. The force fields of non-symmetric substances are also non-symmetric and therefore will interact differently with each of these circularly polarized radiations - each component will propagate at a different speed. In other words, an optically active medium has a different refractive index for left-handed and right-handed polarized light. By recomposing them after passing through the substance, we get plane-polarized light again, but the plane of polarization will be rotated. It can be shown that the angle through which it twists can be simply expressed by the relation

$$\alpha=\frac{180 \cdot l \cdot \Delta n}{\lambda}$$

where α is the angle in degrees, l is the thickness of the optically active medium layer, Δn is the difference in refractive indices for left-handed and right-handed circularly polarized light, and λ is the wavelength of the transmitted light. We can measure the optical rotation of substances using a polarimeter. Monochromatic light (e.g. from a sodium discharge lamp) passes through a polarizing filter (polarizer). Polarized light passes through a cuvette filled with the measured sample. The optical activity of the substance is then evaluated using a second polarizing filter (analyzer), which is fixed in a rotating sleeve. Depending on the position of the analyzer and the plane of the polarized light passing through the sample, the intensity of the light in the eyepiece of the device changes. After finding the rotation of the analyzer at which the intensity of the transmitted light is the greatest, the relative position of both polarizing filters is read on the scale. To facilitate the work of some devices, the light that has passed through the entire system falls on only part of the field of view. The rest of the field of view is illuminated by radiation that did not pass through the polarizer (but did pass through the sample and analyzer). If the analyzer is set correctly, both parts of the field of view light up equally.