Incident light beam

Define Iq to be the intensity of the light incident upon the sample and I to be the intensity of the beam after it has interacted with the sample. The goal of the basic inftared experiment is to determine the intensity ratio I/Iq as a function of the frequency of the light (w). A plot of this ratio versus the frequency is the infrared spectrum. The inftared spectrum is commonly plotted in one of three formats as transmittance, reflectance, or absorbance. If one is measuring the fraction of light transmitted through the sample, this ratio is defined as... 

If the sample is fully or partially transparent to the incident beam, light may be scattered from the back of the sample or from within the sample, and the surface measurement will be inaccurate. 

Figure 15.8. Light transmission of acrylic polymer (5 in thick moulded Diakon. Parallel light beam normally incident on surface). (Reproduced by permission of ICI)...

When a beam of light strikes small particles in its path, the light is said to be scattered the particles send off the light incident on them in many directions. Part of a beam incident on a translucent material, for example. 

The electrochemical cell used in our laboratory has been fully described elsewhere (5). The cell body is made of chemically inert Kel-F and the electrode is mounted on a piston so that its surface can be pushed to the optical window, to a spacing of the order of 1-3 microns, in order to minimize the signal from the bulk electrolyte. For Raman scattering spectroscopy the window is of flat fused quartz, and the exciting laser beam is incident at about 60°. The scattered light is collected off-normal, but the geometry is not critical for SERS due to the high sensitivity. Details on the SERS measurements in our laboratory have been reported previously (6,7). 

But the microscopic fat particles suspended in milk have an average diameter in the range 10-7 to 10-5 m, i.e. much larger than k of visible light. A beam of incident light is scattered rather than transmitted by a suspension of particles - a phenomenon known as the Tyndall effect. 

ROA is also manifest as a small circularly polarized component in the scattered beam using incident light with a fixed polarization state (including unpolarized) [1, 13, 18-20]. Within the far-from-resonance approximation, this circular component (SCP ROA) is measured as (/R — IL) / (/R + /L), where /r and /l represent the intensities of the right- and left-circularly polarized components, respectively, of the scattered light, provides equivalent information and identical spectra to the CID measurement (ICP ROA). Other ROA measurement strategies are possible and are described elsewhere [18, 21, 22]. 

In the transmission Raman approach, the laser beam is incident upon one side of the probed object and the Raman light is collected from the opposite side. 

Unlike ORD, circular dichroism (CD) depends upon the differential absorption of the two circularly polarised part-beams of incident light by an optically active compound. The combined action of anomalous ORD and CD is summarised as the Cotton effect. Therefore Cotton effects can be determined both... 

Gas-phase photoacoustic detection of propane in N2 was attempted on a Si chip. In this method, a modulated light beam is incident on the sample. If the wavelength of the modulated light couples to an energy transition in a gas, the gas absorbs the modulated light resulting in periodic gas expansions and contractions, which are manifested as an acoustic wave. This wave can be detected by a microphone. In contrast to conventional absorption spectroscopy, the sensitivity of photoacoustic spectroscopy scales inversely with dimension, and hence this method is favored in the microscale. This is because photoacoustic spectroscopy is a differential technique in which the absorption is measured as the intensity per unit surface area [796],... 

The extinction of a light beam or incident radiation associated with a cloud of particles basically involves a measure of a moment of the particle number-size distribution roughly proportional to the surface concentration. The extinction of a light beam is well-developed basis for semi-quantitative measurement of particles suspended in gases. Devices for these purposes, including smoke photometers, have been available commercially for many years. They may take many forms, one of which is the transmissome-ter, or opacity instrument. 

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