Polarisation of light

In the optical absorption, two different polarisations of light should be considered the electric field is along (parallel or y polarisation) and perpendicular (perpendicular or x) to the axis. Figure 5 shows the energy band of a metallic CNT for flux < )/< )o =0, 1/4 and 1/2 and the process of optical transitions for the parallel and perpendicular polarisations. Some examples of calculated absorption... 

An asymmetric carbon atom is united to four different atoms or groups, Cabcdy and if the valencies of the carbon are directed to the four corners of a tetrahedron, two different tetrahedra, related as an object is to its image in a mirror, are possible. It is not explained why these arrangements should rotate the plane of polarisation of light in opposite directions, and the explanation is still to some extent obscure. ... 

The history of optical isomerism goes back to the year 1815, when the phenomenon was discovered by the French physicist Jean-Baptist Biot [1]. Optical activity was defined as the ability of a substance to rotate the plane of polarisation of light. Some years later, his student, Louis Pasteur [2] proposed that this optical activity of certain organic compounds was a consequence of their molecular asymmetry, that produces non-super-imposable mirror-image structures. A molecule which is not super-imposable on its mirror image is said to be chiral Conversely, a molecule which is superimposable on its mirror image is described as achiral... 

Perhaps the most important property that differentiates solids and liquids is flow. Liquids flow and take the shape of the container, whereas solids do not flow and tend to retain their shape. The optical properties of liquids and some solids can also be quite different. For example, some solids change the polarisation of light whereas liquids do not. With these ideas in mind, it is not surprising that early investigators who found substances which did not neatly fall into these categories noted their findings and began to ask questions. 

An interesting point concerns polarisation effects in the Raman spectra, which are commonly observed in low-dimensional materials. Since CNTs are onedimensional (ID) materials, the use of light polarised parallel or perpendicular to the tube axis will give information about the low dimensionality of the CNTs. The availability of purified samples of aligned CNTs would allow us to obtain the symmetry of a mode directly from the measured Raman intensity by changing the experimental geometry, such as the polarisation of the light and the sample orientation, as discussed in this chapter. 

The polarimeter is an instrument with which the essential oil chemist cannot possibly dispense. The hypothesis, first seriously enunciated by Le Bel and van t Hoff, that substances which contained an asymmetric carbon atom i.e. a carbon atom directly united to four different atoms or radicles) were capable of rotating the plane of polarisation of a beam of polarised light, has now become a fundamental theory of organic chemistry-. The majority of essential oils contain one or more components containing such a carbon atom, and so possess the power of effecting this rotation. In general, the extent to which a given oil can produce this effect is fairly constant, so that it can be used, within limits, as a criterion of the purity or otherwise of the oil. 

Laboratory data from two groups (see Sect. 3.2.4) indicate that chiral amino acid structures can be formed in simulations of the conditions present in interstellar space. The experimental results support the assumption that important asymmetrical reactions could have taken place on interstellar ice particles irradiated with circularly polarised UV light. The question as to whether such material was ever transported to the young Earth remains open. But the Rosetta mission may provide important answers on the problem of asymmetric syntheses of biomolecules under cosmic conditions (Meierhenrich and Thiemann, 2004). 

Consider a linearly polarised monochromatic light ray incident on a metal surface. Such a ray can always be resolved into two orthogonal components and if the plane of reflectance at the metal is chosen as the reference, then these components correspond to S- and P-polarised light, as discussed in the previous section. 

Dust particles appear to become oriented in interstellar magnetic fields, aligning themselves with the field lines to produce preferential absorption of light polarised... 

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