Light-matter interactions INDEX

The total interaction index describing the interaction of light with matter has two parts. One is concerned with the change in intensity of an incident beam as it passes through an absorbance medium, the other derives from the associated change in the speed of light. The former is measured as an absorbance, the latter as the refractive index of a solute in a solvent. 

Equations (10.17) and (10.18) show that both the relative dielectric constant and the refractive index of a substance are measurable properties of matter that quantify the interaction between matter and electric fields of whatever origin. The polarizability is the molecular parameter which is pertinent to this interaction. We shall see in the next section that a also plays an important role in the theory of light scattering. The following example illustrates the use of Eq. (10.17) to evaluate a and considers one aspect of the applicability of this quantity to light scattering. 

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. 

Chirality is the origin of the spectroscopic property optical activity. The interaction of light and matter is characterized by the refractive index and the absorption coefficient. For chiral molecules, both the refractive index and the absorbance coefficient of one enantiomer differ for right and left circularly polarized light (r-cpl and 1-cpl). 

It is probably safe to say that there is really no satisfactory mathematical description that can fully account for all of the possible interactions between light and matter, particularly in complex biological media. Two common interactions are illustrated in figure 4.2. Although Newton incorrectly explained the principle of refraction, it is now understood that light changes speed and direction when passing from one medium to another. The index of refraction is defined as... 

The intensity of scattering can be modeled by the same equations developed for light scattering, provided that account is taken of the different matter-radiation interaction. For visible light, scattering is a result of differences in the refractive index of the solute and the solvent, and the optical constant K (Chapter 9, Equation 9.18) is proportional to n idh/dc). For SAXS, the scattered intensity is a function of the electron density, and the molar mass is then related to the excess electron density Ap of solute over solvent. For A, = 1.54 A, the Rayleigh ratio at 0 = 0, Rq is ... 

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