Becke lines

Bechamp batch process Bechamp reaction Beck and Guthke cell Becke line technique... 

The refractive index, n, may be measured using an optical microscope [1,2,23,27,34]. 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 [1], The Becke line method gives the surface refractive index [1],... 

BECKE TEST. A microscope of moderate or high magnification is used to compare the indices of refraction of two contiguous minerals for of a mineral and a mounting medium or immersion liquid), in a thin section or other mount. When the two substances differ substantially in refractive index, they are separated by a bright line, called the Becke line. The line moves toward the less refractive of two materials when the tube of the microscope is lowered. 

Another test to determine whether the particle or medium has the higher n is the Becke test. The Becke line is a halo around the particle. As the objective is raised or the stage is lowered, the Becke line will move into the medium or particle having the higher n. As the objective is lowered or the stage is raised, the Becke line will enter the medium or particle having the lower n. 

Birefringence measurements were obtained by the Becke line method, and density measurements were obtained by the sink-float method (26). 

CHARACTERIZATION. Melting points were determined on an E. I. DuPont Series 99 Thermal Analyzer at 20°C/min. Inherent viscosities of polyamic acid solutions were obtained at a concentration of 0.5% (w/w) in DMAc at 35°C. Glass transition temperatures (T ) of the fully cured polymer films were measured by thermomechanical analysis (TMA) on a DuPont 943 Analyzer in air at 5°C/min. Films fully-cured at 300°C were tested for solubility at 3-5% (w/w) solids concentration in DMAc,N,N-dimethylformamide (DMF), and chloroform (CHCl-j). Solubilities at room temperature were noted after periods of 3 hours, 1 day and 5 days. Refractive indices of 1 mil thick films were obtained at ambient temperature by the Becke line method (11) using a polarizing microscope and standard immersion liquids obtained from R. P. Cargille Labs. 

One of the more useful methods is the Becke test. Becke noticed that when the microscope is focused up and down a bright halo near the boundary of a particle moves in and out. The halo will always move toward the higher refractive index as the focus is raised and toward the lower refractive index as the focus is lowered. Thus if the particle has a refractive index higher than the liquid, the Becke line will move from outside the boundary to the inside as the focus is raised and vice versa. Figures 4 and 5 show the bright Becke line outside and inside the boundary, respectively. 

HG. 5—Becke line inside particles with raised focus showing that particles have a higher refractive index than liquid. 

ISO R489 suggests two methods based on Abbe refractometer and the Becke line methods. It does not recommend any specimen conditioning procedure prior to the test. DIN 53491 provides practical details relevant to refractometer measurements. It recommends test temperature... 

Brewster s law. Reflected light in general is more polarised than the incident light and according to Brewster s law, the polarisation is maximum when i + r = 90 where i and r are the angles of incidence and refraction. There are several methods described in the literature such as those based on the use of prisms and fluid immersion (in fluids of known refractive indices) and index matching. The so-called Becke line technique for index matching uses observations under microscope. 

General methods of measuring refractive index, not confined to plastics, can be found in older textbooks on optics [3]. Well-known methods arc those depending on matching the unknown solid with known liquids, using the Becke line technique [4], dispersion staining [5], techniques based on the Abbe refractometer [3], and the measurement of the ratio of real depth to apparent depth [3]. 

The two methods recommended in ISO 489 [6] are the Abbe refractometer technique and the Becke line technique. 

The material under test is mounted in a liquid of known refractive index and examined in monochromatic light with the condenser adjusted to give a narrow axial beam. When the test pieces and the liquid have different refractive indices, each particle is surrounded by a narrow luminous halo (the Becke line), which moves as focus is adjusted. If the focus is lowered, then the Becke line moves towards the medium having the lower refractive index. The test is repeated with particles mounted in other immersion liquids until a match is found or until the index of the test sample lies between two known indices in the series of liquid standards. If the Becke line phenomenon does not appear, then the refractive index of the material being examined is equal to that of the immersion liquid. 

Fibers sometimes present difficulty in that there is a different orientation at the skin and in the core of the fiber. In these cases it must be borne in mind that the Becke line... 

Using a Berek compensator a value of 0 65 was found. This was felt to be too high and an attempt was made to determine the individual refractive indices using a Becke line assessment with immersion liquids of different refractive indices. The refractive index perpendicular to the fibre axis, was found to be in the range 1-63-1-64. The refractive index /."n parallel to the fibre axis, was very much more difficult to measure but... 

Using the Becke line (or other comparative re fractive-index techniques) determine whether the particle has an index higher or lower than the RI of the liquid. Wash off the liquid and cover glass with a gentle stream of xylene over a container that can be sealed for safe storage and disposal. 

Becke line. A band of relatively high-intensity light, visible around or in nonopaque particles observed under the microscope apparent movement of the band during focus adjustment relates to differences in indices of refraction between the particle and adjacent liquid or solid medium. 

The two measurement methods recommended by IS0489 are the Abbe refractometer technique and the Becke line technique. The latter is more useful with powdered or granulated transparent material or even with small chips of material taken from a larger sample. ASTM D542 includes only the Abbe method and it is widely used for characterization of refractive index of polymers. 

Becke test n. The method for determining refractive index of a transparent particle by noting the direction in which the Becke line moves. The halo (Becke lines) will always move to the higher refractive index medium as the focus is raised. The halo crosses the boundary into the lower refractive index medium when the microscope is focused down. The particle must be illuminated with a narrow cone of axial light obtained by closing the aperture diaphragm of the condenser to a small aperture. Nesse WD (2003) Introduction to optical mineralogy. Oxford University Press, New York. 

False Becke line n. A second bright line, which moves in the direction opposite to the Becke line. It is usually observed with thick particles or when the refractive index difference between particles and mountant is large. It appears as a concentration of reflected light at the low index side of the interface. 

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