Fresnel reflection

In employing a thin-layer configuration the external reflectance approach reduces the problem of the strong solvent absorption in two ways. Firstly, this configuration yields a solution layer only a few microns thick. Secondly, exact calculations employing the Fresnel reflection equations show that the radiation absorbed by an aqueous layer c. 1 urn thick in contact with a reflective electrode is attenuated to a lesser extent than would be predicted by the Beer - Lambert law. 

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 necessary pump powers can be achieved either by other lasers (e.g. nitrogen lasers, solid-state lasers or even focussed He-Ne- or Ar+-gas lasers) or by flash-lamps. The simplest practical arrangement is a square spectrophotometer cell, polished on all sides, containing the dye solution which is pumped by a nitrogen laser whose beam is focussed into a line parallel to and directly behind one of the cell windows. Then the Fresnel reflection from the two adjacent windows gives enough feedback in most cases, so that no additional resonator mirrors are needed and the dye laser oscillation starts. 

The Fresnel reflection coefficients r for the overall system can be obtained from the elements m of the characteristic matrix M associated with the overall system ... 

The overall reflectance R for parallel- and perpendicular-polarized light can be calculated from the Fresnel reflection coefficient ... 

Fresnel Reflection—The reflection of a portion of the light incident between two homogeneous media having different refractive indices. Fresnel reflection occurs at the air/glass interfaces at entrance and exit ends of an optical fiber. 

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. 

Here, n and n2 are the refractive indices of the interfacing materials. The surface reflectivity R, Equation 5, used by Robinson is the Fresnel reflectivity for normal light incidence (c.f. 4). Robinson s interpretation explains the well-known linear relationship between the specific surface area of a paper, as measured by gas adsorption, and its scattering coefficient (15. 28, 32). It also rationalizes the observed decrease in scattering and brightness of wet compared to dry pulp (33, 34). 

The Kubelka-Munk theory of diffuse reflectance is a good description of the optical properties of paper. The two parameters of the theory, absorption and scattering coefficient, are purely phenomenological, but are closely related to basic properties of paper. The absorption coefficient is approximately a linear function of the chrcmgphore concentration in the paper. The scattering coefficient is related to the nonbonded fiber surface area in the paper, or the area "not in optical contact," and the Fresnel reflectivity of that surface. 

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As in any other laser, the lasing threshold in a semiconductor laser diode is reached when the gain of the active material overcomes the losses of the laser cavity. These losses have two basic origins, namely the finite reflectivity of the mirrors mid distributed losses due to scattering and parasitic absorption in the active medium. In contrast to other lasers, the mirrors in typical semiconductor lasers are simply formed by cleaved or etched crystal facets. Therefore, the reflectivity (Fresnel reflectivity) is rather low, about 20% in the case of the nitrides. 

Ellipsometry grants the independent determination of two results per resolution element without changing the experimental geometry this technique is described in Sec. 6.4.4.2. More often however, the information on the phase shift is abandoned and just the reflectance is determined. Usually it is measured by comparing the signal caused by the reflected intensity with the one obtained when the sample is replaced by a mirror. Even without correcting for its reflectance such results can often be used for further mathematical evaluation. The Fresnel reflection coefficient r in the form given by Eq. 

Fresnel reflection Eqs. 6.4-4 apply directly in their most simple version. 

The linearity of the K-M function has been investigated for several different conditions. For caffeine, as diluted in KCl powder and reaching up to 100% of weight, maxima of weak and strong absorption bands were tested for this important analytical parameter. It was found that under the conditions given, the use of an accessory with off-axis optical geometry and with application of crossed polarizers before and after the sample, reduced contributions from specular Fresnel reflection from particles on the upper sample surface. As a result, the linear region... 

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