Wavelength, light, measurement

Polarimetry. Polarimetry, or polarization, is defined as the measure of the optical rotation of the plane of polarized light as it passes through a solution. Specific rotation [ a] is expressed as [cr] = OcjIc where (X is the direct or observed rotation, /is the length in dm of the tube containing the solution, and c is the concentration in g/mL. Specific rotation depends on temperature and wavelength of measurement, and is a characteristic of each sugar it may be used for identification (7). 

Finding the concentration of an unknown solution is an important procedure in laboratory work. One method commonly used to determine solution concentration is to measure how much of a single wavelength of light is absorbed by the solution and compare it to known values of concentration and wavelength. Light absorbance is directly related to the concentration of a solution. This relationship is called Beer s law. 

Figure 19. Configuration for experimental reflectivity measurements on a film using a fixed or variable wavelength light source. The film thickness (d) is indicated as well as the refractive indices of the plasma (n ), film (nx), and substrate Oii)-...

Light emitted from the source converges on the excitation monochromator, which allows a narrow band of wavelengths to be selected (15 nm) to induce fluorescence of the sample solution in the measurement cell. The emitted light, observed perpendicular to the direction of the incident beam, passes through the emission monochromator, allowing the selection of a narrow band of wavelengths for measurement (Fig. 12.9). The simplest instruments have a double compartment for measurement. This allows the sample solution and a standard fluorescent reference solution to be put into the optical path. 

The light then enters a polychromator, which disperses the light into its component wavelengths and directs the light at the diode array. Each diode receives a different wavelength, and all wavelengths are measured simultaneously. Resolution depends on how closely spaced the diodes are and how much dispersion is produced by the polychromator. The spectrum in Figure 21-26 (in the next chapter) was produced with a photodiode array detector. 

Optical rotations usually are measured at just one wavelength, namely 589.3 nm, simply because sodium-vapor lamps provide an especially convenient source of monochromatic light. Measurements at other wavelengths are less easily made without specialized instruments, with which relatively few laboratories are currently equipped. Nevertheless, much information has been obtained about structure, conformation, and configuration of organic compounds from measurements of optical rotation as a function of wavelength (i.e., optical rotatory dispersion). 

V is called the Verdet-constant it depends only slightly on the temperature, but varies markedly with the wavelength of the light measurements are normally carried out... 

---