Fresnel refractive index

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

Specular reflection of electromagnetic radiation at the (electrochemical) interface is generally described by Fresnel equations. Supposing the most simple case that both the electrolyte and electrode are transparent and differ only in their refractive indexes, nx and n2, the reflectivity for normal incidence of the radiation equals ... 

The approximate picture of a transparent electrochemical interface is rather far from the actual situation. Nevertheless, the Fresnel equations are valid also for absorbing media, if one uses the complex refractive indexes, Nk ... 

To evaluate its capability for refractive index measurement, the fiber FPI device was tested using various liquids including methanol, acetone, and 2-propanol at room temperature. The interference spectra of the device immersed in various liquids are shown in Fig. 7.12 for comparison. The signal intensity dropped when the device was immersed in liquids as a result of the reduced refractive index contrast and thus lowered Fresnel reflections from the cavity endfaces. However, the interference fringes maintained a similar visibility. The spectral distance between the two adjacent valleys also decreased, indicating the increase of refractive index of the medium inside the cavity. Using (7.4), the refractive indices of the liquids were calculated to be nmethanoi = 1 -3283, acetone = 1 -3577, and n2-propanoi = 1.3739, which was close to the commonly accepted values. 

The opacity of plastic foams, and polymers with scratched surfaces, is also governed by Fresnel s law. The n value of the gas which occupies the scratch indentation is much lower than that of the polymer. Light may be directed through rods of transparent polymers, such as PMMA. This effect may be enhanced when the rod or filament is coated with a polymer with a different refractive index, such as polytetrafluoroethylene (ptfe). Optical fibers utilize this principle. 

It is possible to obtain A and / fromEq. (7.118).55 However, the extraction of the refractive index of the film and its thickness involves Fresnel s equation for the interaction of light with matter, and this mathematical manipulation was impractically laborious before the introduction of computers in the 1960s. 

Fresnel Refection losses—Reflection losses that are incurred at the input and output of optical fibers due to the differences in refraction index between the core glass and immersion medium. 

Index Matching Material—A material, often a liquid or cement whose refractive index is nearly equal to the core index. Used to reduce Fresnel reflections from a fiber end face. 

The Waters Assoc. R401 differential refractometer is based on an optical-deflection design rather than on light reflection. This permits the use of a single cell throughout the refractive-index range of 1.00-1.75. The system also has a wider dynamic range of linearity for quantitation than the Fresnel-type refractometers. 

The refractive index n is not regarded as a constant as in Fresnel s equation, but as an abbreviation for k c/co. This can be understood as follows in an optical medium the phase velocity of light is c/n, where c is the velocity of light in the vacuum. Now the phase velocity is connected with wavelength and frequency through the relation c/n = v. On the other hand k = 27t/A. This leads to c/n = u/k and finally... 

Refractive Index. Commercial LC refractometers are based on one of two designs, deflection or Fresnel, as shown in Figure 9.28. Both types require reference and sample cells typical of bulk property detectors. 

Figure 9.28. Optical diagrams for refractive index detectors for LC (a) Fresnel type by permission of LDC/Milton Roy (h) deflection type by permission of Millipore Corporation, Waters Chromatography Division.

Combination of Surface Plasmon Resonance (SPR) and Optical Waveguide Spectroscopy (OWS) was used for the simultaneous determination of refractive index and film thickness of the hydrogel layers in the Kretschmann configuration [24], The resulting angle scans from the SPR instrument were fit to Fresnel calculations and different layers were represented using a simple box model. A detailed description of this process has been published previously [18]. 

The film thickness of hydrogel layers from 5 wt % solutions was in the order of 1 jun in the swollen state. Thickness (d) and refractive index (q) were determined without further assumptions, because these films could be fitted using a single layer model [19], The refractive index (q) obtained from Fresnel calculation was converted to polymer fraction and volume degree of swelling (1/0P). 

In the simplest case of the interface between two bulk media, the reflectivity is related to the refractive index difference across the interface, and is described by Fresnel s Law [18]. The refractive index, n, is related to the neutron scattering length density, such that... 

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