Light helical medium

A plot of the optical absorbance at 260 nm (the wavelength of maximum light absorption by nucleic acids) versus temperature is known as a melting curve (Fig. 5-45). The absorbance is lower, by up to 40%, for native than for denatured nucleic acids. This hypochromic effect (Chapter 23) is a result of the interaction between the closely stacked bases in the helices of the native molecules. The melting temperature Tm is taken as the midpoint of the increase in absorbance (Fig. 5-45). As the percentage of G + C increases, the nucleic acid becomes more stable toward denaturation because of the three hydrogen bonds in each GC pair. Tm increases almost linearly with increases in the G + C content. In the "standard" citrate buffer (0.15 M NaCl + 0.015 M sodium citrate, pH 7.0) Eq. 5-22 holds. The exact numerical relationship depends strongly upon the ionic composition and pH of the medium.37 72 552 553... 

Initial experiments with a conventional luminescence microscope equipped with the relevant filters point to the [Ln2(L13)3] helicates penetrating into HeLa cells in a concentration- and time-depending manner. In Figure 105 (top), the Eu luminescence is plotted versus the concentration of the [Eu2(L13)3] helicates in the culture medium after an incubation time of 7 h at 37 °C. The luminescence emitted by the [Eu2(L13 )3] chelate at all concentrations is understandably smaller than the light emitted by the two other helicates, in view of it smaller quantum... 

Now if a medium were related to one of the circular components of a plane polarized light wave as the female thread to the male thread of a helical screw, it would be expected to transmit that one more easily than the oppositely directed one, and so cause a rotation of the plane of polarization. We have therefore to examine the conditions under which the analogy of the screw might be applicable. Some crystalline media, for example certain forms of quartz, do in fact possess a space lattice with a helical configuration of atoms, and the successive actions of these on the light waves can be imagined to produce retardations depending on the direction of the circular component. 

Fig. 12.6 Dispersion relation for a cholesteric that has helical structure with wavevector qo. The abscissa corresponds to the wavevectors of the eigenmodes propagating in the medium and the ordinate is the frequency of the incident light proportional to its wavevector, k = (a) Very...

We showed in last section that in a uniform anisotropic medium, for each propagation direction, there are two eigenmodes which are linearly polarized. The polarization state of the eigenmodes is invariant in space. In this section, we discuss the propagation of light in a special case of a non-uniform anisotropic medium a cholesteric liquid crystal which locally is optically uniaxial, but the optic axis twists uniformly in space [6,7]. Choose the z axis of the lab frame to be parallel to the helical axis of the cholesteric liquid crystal. The pitch P of the liquid crystal is the distance over which the liquid crystal director twists In. The components of the liquid crystal director of a right-handed cholesteric liquid crystal q > 0) are given by... 

We point out that propagation of light waves along the helical axis in such a medium is analogous to propagation of electron waves in a one-dimensional periodic lattice. What we have here are Bloch pho-... 

Most frequently, a planar orientation of CLCs is obtained by the rubbing technique. However, in cells coated with polyamide (without rubbing), a new cholesteric state, called an amorphous cholesteric structure, is formed [82]-[85]. The surface, covered by polyamide, orients the molecules parallel to the surface, but without any preferable direction in the surface plane. Thus, a structure with random orientation of liquid crystal molecules, and a helical axis oriented normal to the surfaces, is obtained (Figure 6.2(e)). All parts of the amorphous cholesteric structure with medium chirality rotate the polarization plane by the same angle. The whole structure can be considered as optically active. In this case, the angle of rotation is independent of the direction of the polarization plane of the incident light. This is the basic property used for electro-optic application of the amorphous cholesteric structure vrith medium chirality. Characteristic features of this effect are ... 

Here, p is the pitch of the helical structure. An is the birefringence of the medium and n is the average refractive index, (rio+n ll. The temperature range over which the cholesteric material selectively reflects visible light is known as the color play of the material. The temperature dependence of the... 

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