Showing papers by "S. Chandrasekhar published in 1953"

Theoretical interpretation of the optical activity of quartz

Abstract: a-QUARTZ belongs to the trigonal enantiomorphous hemihedral class of symmetry, the space-group being represented by D34 (of the enantiomorphous D39. The structure accordingly has a threefold screw axis. The unir cell consists of three SiO2 triplets spaced at equal intervals along the vertical axis, each turned through 120 ~ with respect to its predecessor. It can scarcely be doubted that this screw structure is responsible for the optical rotatory power of quartz. Theories whŸ seek to correlate the optical activity with the ultimate structure of the substance exhibiting ir have been put forward by various authors in the past (vide the reviews by Condon, 1937; Kauzmann et al., 1940). Applications of these theories to evaluate the results in particular crystals have been comparatively few. For the case of quartz, there have been three attempts to calculate the rotatory power theoretically. The first is by Hylleraas (1927), who has applied Born's general theory of coupled oscillators (1915). Hylleraas has deduced from the known data for the double refraction and the rotatory power, the values of two lattice constants, which agree well with the X-ray data. The second attempt is by de Mallemann (1930) who has estimated the rotatory power in terms of the refractivities of the atoms constituting the crystai. More recently, Ramachandran (1951) has made detailed ca!culations of the first order terms of the polarisability theory and obtains good values for the rotatory power for the propagation of light both along a n d a t right angles to the optic axis. Though all these authors get a fairly good agreement with the observed values, none of them has derived ah explicit law of rotatory dispersion for quartz, which is certainly a characteristic and noteworthy feature of the phenomenon.

A redetermination of the rotatory dispersion of sodium chlorate

Abstract: SODIUM chlorate is the best known exarnple of a crystal belonging to the cubic system exhibiting optical activity. As an aqueous solutiorl of it does not display such activity, it is clear that the optical rotatory power is a consequence of the crystal structure which belongs to the space group T4-p213. Precise data regarding the rotatory power and its variation with wave-length are obviously needed a s a starting point for any theoretical discussion of the optical behaviour of the crystal in relation to its structure. Determinations have been made in the visible region of the spectrum by Sohncke (1878), Voigt (1908), Perucca (1919) and also by Ramaseshan (1948). Measurements extending far into to the ultra-violet were first made by Guye (1889) and later by Rose (1909), both of whom have obtained values of the rotation from 7200 A.U. to 2500 A.U. Though it is known that sodium chlorate transmits light upto nearly 2200 A.U., it appears that no one has since pursued the measurements farther into the ultra-violet. The data obtained by Guye and Rose, which are the most extensive, have been presented in Table I. From the data it can be seen that in the visible region Biot's Law is approximately valid, but as we proceed into the ultra-violet, the rotatory power increases at a much slower rate than is required by Biot's Law.