Measurement of Refractive Index

Refractive index n is easily measured in the optical microscope and helps to identify unknown materials [5,9,11,12]. Small particles are mounted in a liquid of known refractive index and observed in transmission. The liquid can be changed until (liquid) = (particle), when a transparent or translucent particle will have very low contrast. To determine whether the change should increase or decrease n, the 

Phase contrast increases the contrast due to differences in n, and allows a more accurate determination. Interference contrast in transmission gives the optical path length and the average refractive index through the specimen thickness. The Becke line method gives the surface refractive index. Fibers often give different results by the two methods because of the variation in refractive index across a fiber cross section. 

Phase contrast devices increase the contrast due to differences in refractive index and so 

Slides containing sections are washed in a saturated solution of sodium hydroxide in ethanol for 5 min to remove the embedding plastic from the sections. After careful washing in ethanol and drying, the sections are ready for measurement. 

The section is first mounted in ethanol. The instrument is adjusted to give the maximum extinction position (m.e.p.) of the background (Fig. 4.3.2) by tilting the crystal plate of the beam splitter in the condenser (Fig. 4.3.1). Measurements of analyzer rotation are made by rotating the analyzer filter until the area of the specimen to be measured reaches its m.e.p. (Fig. 4.3.2). 

The angle is read off a vernier scale on the control knob. This procedure is repeated several times and an average value is calculated. This value is converted to an o.p.d. value using Eq. (1). The coverslip is then removed and the ethanol allowed to evaporate. The section is remounted in glycerol and the same point on the same section is remeasured. Refractive index can be calculated using Eq. (2) where Nj and N2 are 1.362 and 1.474 for ethanol and glycerol, respectively. All measurements are made at 20°C unless otherwise stated. 

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Wise, J.L. and L.C. Chhabildas (1986), Laser Interferometer Measurements of Refractive Index in Shock-Compressed Materials, in Shock Waves in Condensed Matter (edited by Y.M. Gupta), Plenum, New York, pp. 441-454. 

Miniaturized and robust optical fiber sensors capable of accurate and reliable measurement of refractive index of the surrounding environment have attracted tremendous interest in recent years. One of the driving forces for the development of these fiber optic devices is their broad applications in chemical sensing. When placed in the liquid solution or gas mixture, these fiber sensors can detect the chemical composition change by monitoring its refractive index variation. These... 

Ran, Z. L. Rao, Y. J. Liu, W. J. Liao, X. Chiang, K. S., Laser micromachined Fabry Perot optical fiber tip sensor for high resolution temperature independent measurement of refractive index, Opt. Express 2008, 16, 2252 2263... 

Kim, Y. Paek, U. Lee, B. H., Measurement of refractive index variation with temperature by use of long period fiber gratings, Opt. Lett. 2002, 27, 1297 1299... 

The cell is fitted with windows, and the concentration of polymer along its length is determined by optical methods which are based on measurements of refractive index of absorption solvents to be used for ultracentrifugation experiments must be chosen for difference from the polymer in both density and refractive index. An effort be made to avoid mixed solvents. Low solvent viscosity is also desirable. 

A quantitative measure of interaction between polymer and monomer in solution can be expressed by a value of preferential solvation. Preferential solvation can be calculated from the measurements of refractive index increments in dialysis experiments. This experiment can be illustrated as in Figure 3.1. 

For polarized IR spectroscopy, one must consider crystal optics. If the crystal is birefringent, the optical disturbance is usually perturbed in some cases it is not even perpendicular to the direction of propagation. This could pose severe problems with some crystals, but our measurements of refractive index... 

Wood which has been hydrolyzed in 72% sulfuric acid for 3h at 70°C is washed exhaustively in water, dehydrated in acetone, and embedded in Spurr resin. Measurements of refractive index are made in the same way as described above. In addition, an assessment of porosity is carried out as follows For sections mounted in glycerol, o.p.d. is measured in the regions to be compared, e.g., S3 and S2. These measurements are then converted to effective thickness values using the following equation ... 

Thus the experiment consists of two parts the measurement of the dielectric constant of the gas by a method similar in principle to that of Exp. 29, and the measurement of the refractive index with an interferometer. The measurement of a dielectric constant at more than one temperature constitutes a complete stand-alone experiment, independent of the refractive index measurement the measurement of refractive index can be used as a standalone experiment if a value for the low-frequency dielectric constant is supplied to permit the calculation of the dipole moment. 

Many relatively slow or static methods have been used to measure Tg. These include techniques for determining the density or specific volume of the polymer as a function of temperature (cf. Fig. 11-1) as well as measurements of refractive index, elastic modulus, and other properties. Differential thermal analysis and differential scanning calorimetry are widely used for this purpose at present, with simple extrapolative eorrections for the effects of heating or cording rates on the observed values of Tg. These two methods reflect the changes in specific heat of the polymer at the glass-to-rubber transition. Dynamic mechanical measurements, which are described in Section 11.5, are also widely employed for locating Tg. 

Appraisals of the optical properties of lipid solutions and dispersions will provide information on concentrations, aggregation and stability, phase transitions, densities, and repeating structuresJ Measurements of refractive index, scattered light intensity (polarized and depolarized), and birefringence are relatively easy laboratory methods on which certain product specifications may be based. Also, fluorescent techniques can readily provide information on lipid movements and transfer of lipid between particles. ... 

For the rapid determination of the quantity of glue or gelatin in dilute liquors for the control of manufacturing processes physical methods are essential. Measurements of refractive index have been found very suitable. ... 

Chong J, Shum P, Haryono H, Yohana A, Rao M, Lu C, Zhu Y (2004) Measurements of refractive index sensitivity using long-period grating refractometer. Opt Commun 229 65-69... 

Two methods (ASTM D-1218, ASTM D-1747) are available for measuring the refractive index of viscous liquids. Both methods are limited to lighter-colored samples for best accuracy. The latter test method (ASTM D-1747) covers the measurement of refractive indexes of light-colored residual fuel oil at temperatures from 80 to 100°C (176-212°F). Temperatures lower than 80°C (176°F) may be used provided that the melting point of the sample is at least 10°C (18°F) below the test temperature. This test method is not applicable, within reasonable standards of accuracy, to liquids having darker residual fuel oil (having a color darker than ASTM Color No. 4 ASTM D-1500). 

Two general type of detection devices are available. These are bulk property detectors and solute property detectors. The bulk property detector measures a change in some overall physical property in the mobile phase as it emerges from the column. Two typical examples are measurement of refractive index and conductance. The solute property detector is sensitive to changes in a physical property of the solute as it emerges from the column in the mobile phase a typical example is the measurement of ultraviolet and / or visible absorption. In general,... 

Onofri, F., Girasole, T., Grehan, G., Gouesbet, G., Brenn, G., Domnick, J., Xu, T.H. and Tropea C., Phase-Doppler anemometry with the dual burst technique for measurement of refractive index and absorption coefficient simultaneously with size and velocity. Part. Part. Syst. Charact., 13, 112-124 (1996)... 

ASTM D542 describes two methods, viz., i) refractometric and ii) microscopic, for the measurement of refractive index of transparent organic plastics. Both the methods require optically homogeneous specimens of uniform index. The ASTM recommends that refractometric method is to be... 

An accurate measurement of the concentration of the solute in the continuous phase can be obtained by sampling and evaporating to dryness, but this is, of course, prohibitively time-consuming for control purposes. There are several methods for the rapid measurement of continuous-phase concentration. The measurement of refractive index has been shown to provide quick and accurate concentration determination (Kelt and Larson 1977 Mullin and Leci 1972 Sikdar and Randolph 1976). Garside and Mullin (1966) suggested the use of an on-line densitometer to determine solution concentration, and Witkowski (1990) successfully used densitometry for concentration measurements to estimate the kinetic parameters of an isothermal crystallizer. It should... 

All the quantities in equation (9) are susceptible of direct measurement except v, the number of molecules per cc. This can, therefore, be deduced from measurements of refractive index and of turbidity, if the wavelength of the light employed is known. 

Amplitude-based SPR sensing devices typically attain a refractive index resolution in the 10 —10 range. Angular and spectral interrogation-based systems provide even better resolution, allowing the measurement of refractive index changes as small as 10 . This corresponds to a detection limit for an adsorbed protein layer of less than 1 pg mm . ... 

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