• A K Ramdas

Articles written in Proceedings – Section A

• Thermo-optic behaviour of silicon carbide

The dispersion and birefringence of carborundum are well represented by the formulæ$$\begin{gathered} \omega ^2 = 1 + \frac{{k_1 \lambda ^2 }}{{\lambda ^2 - \lambda _1 ^2 }} + \frac{{k_2 \lambda ^2 }}{{\lambda ^2 - \lambda _2 ^2 }} \hfill \\ \varepsilon ^2 = 1 + \frac{{k_3 \lambda ^2 }}{{\lambda ^2 - \lambda _1 ^2 }} + \frac{{k_4 \lambda ^2 }}{{\lambda ^2 - \lambda _2 ^2 }}, where \hfill \\ \lambda _1 = 155 m\mu , \lambda _2 = 175 m\mu , k_1 = 3 \cdot 657, k_2 = 1 \cdot 898, \hfill \\ k_3 = 1 \cdot 626, k_4 = 4 \cdot 11. \hfill \\ \end{gathered}$$ The theory of thermo-optic behaviour of solids when applied to the case of carborundum explains the observed positive temperature coefficient of both the indices.

• The thermo-optic behaviour of quartz

The temperature variation of the refractive indices ofα-quartz contains a positive part, though it is completely submerged by the predominant negative part due to the thermal expansion. At theα-β transition temperature of 573°C., the structural changes represented by theα-β transformation cause a change in a direction opposite to that of the density change,i.e., an increase in the refractive indices. The rate of change of the electronic dispersion wave-lengths decreases with decreasing temperatures tending to zero at very low temperatures. The thermo-optic behaviour of quartz and vitreous silica bear a close resemblance, the difference in the two cases being due to the difference in the thermal expansions.

• The infra-red absorption spectrum of potassium chlorate crystals: Part I

The infra-red transmission through the 001 face of a single crystal of potassium chlorate reveals, besides a broad absorption band in the range 9·2µ to 12µ, a second and narrower band with its maximum at 5·3µ. The former band can be explained as due to the infra-red activity of the vibration frequenciesν1 (930 cm.−1) andν2 (975 cm.−1) of the crystal also observed in the Raman effect, and their combinations with the low frequency lattice oscillations. The band at 5·3µ may likewise be explained as due to the infra-red activity of the octaves ofν1 andν2 and of their summational frequency.

• The infra-red absorption spectrum of potassium chlorate crystals—Part II

Crystals of potassium chlorate exhibit strong absorption bands in the infra-red at 10µ–10·8µ, and at 16·2µ and 20·3µ. These are identified with the active fundamentals atω1 (930 cm.−1, 10·75µ),ω2 (975 cm.−1, 10·26µ),ω3 (620 cm.−1, 16·13µ) andω4 (486 cm.−1, 20·58µ). It is noticed thatω3 is quite strong, while the corresponding Raman frequency comes out very weakly. The vibrationω1 is more strongly Raman-active thanω2, while the reverse appears to be the case in the infra-red. Absorptions are also observed at 5·3µ, 6·3µ and 6·9µ which are interpreted as first-order combinations among these four fundamental frequencies, while other bands at 13·5µ, 18µ and 18·75µ appear to be combinations of internal frequencies with low frequency lattice vibrations.

• The infra-red absorption spectra of sodium nitrate and calcite

The infra-red absorption spectra of sodium nitrate and calcite crystals have been studied in the range 1 μ to 22 μ with a Beckman infra-red spectrophotometer, using three different thicknesses of a cleavage section in each case. The thinner plates exhibit the structure of the more intense absorption bands while the thicker ones show their harmonics as well as the weaker bands very clearly. The remarkable sharpness of the harmonics is a noteworthy feature. Much new detail has been observed with sodium nitrate in the region of wavelength greater than 8 μ. A strong absorption in the region beyond 17μ has been found in both crystals. This is ascribable to harmonics of lattice oscillations. The entire body of data is satisfactorily explained in terms of the normal modes of vibration jointly of the two units (NaNO3 or CaCO3) contained in the unit cell of the crystal lattice.

• The infra-red absorption spectra of sodium chlorate and potassium chlorate crystals

• The infra-red absorption spectrum of silicon carbide

The absorption spectra of crystals of silicon carbide, studied in the range of 1μ to 20μ, reveal besides an intense region of absorption from 10μ to 14μ in which the feflexion maximum at 12μ falls, several comparatively well-defined absorption maxima at shorter wavelengths,viz., at 3·5μ, 5·0μ, 6·3μ to 6·5μ, 7·2μ and 7·7μ in a typical case.

Silicon carbide exists in various crystal forms. From a group theoretical analysis applied to a few typical cases it emerges that the normal modes of the uniaxial silicon carbide consist of movements of silicons and carbons along the optic axis or normal to it.

The reflexion maximum at 12μ, and the Raman line at 818 cm.−1 (12·2μ) can be attributed to an oscillation in which carbons and silicons vibrate in opposite phases. The existence of numerous other vibrations besides this one accounts for the extension of the absorption band beyond 12μ. The absorption maxima in the short wavelengths can be attributed to first order and second order combinations and overtones of these fundamentals.

• The infra-red absorption spectrum of barytes

The infra-red absorption spectrum of barytes has been studied in the range 1 μ to 25 μ using ten different specimens, belonging to 001 and 110 sections. The extension of the range of study beyond 16μ has revealed the presence of fundamentals at 21 · 3 μ, (470 cm.−1) and 22 · 8 μ (439 cm.−1) besides other absorption maxima. The review of the available data shows that of the eighteen Raman active internal oscillations of barytes, fourteen can be identified including the one at 967 cm.−1 not previously recognised as such. The infra-red studies give ten of the fifteen theoretically possible infra-red active internal oscillations. The numerous absorption bands observed in the range studied find satisfactory explanation in terms of the infra-red active fundamentals in conjunction with the Raman active ones, applying selection rules appropriate to the unit cell of the crystal.

• On the polycrystalline forms of gypsum and their optical behaviour

The paper brings to notice the remarkable optical effects exhibited by a polycrystalline form of gypsum which is different from both alabaster and satin-spar in its structure. It is not a fibrous material but consists of fine rods orientated nearly parallel to theb-axis of gypsum and exhibits a ready cleavage along planes perpendicular to that axis. A source of light viewed through a plate of the material exhibits, in general, three concentric circles which are polarised in a characteristic fashion. The source itself appears as a luminous point on the second or middle circle. It is shown that these circles arise by reason of the reflection of light at the boundaries between the rod-like crystals composing the material, for which the name “fascicular gypsum” is accordingly proposed. A theoretical explanation of the phenomena is given and photographs of the same are reproduced. Observations on the optical behaviour of alabaster and of satin-spar are also reported.

• The structure and optical behaviour of iridescent calcite

Arrays of multi-coloured images of a distant light-source arranged in geometric patterns are exhibited by calcite rhombs traversed by twinning layers in several directions simultaneously. The paper reproduces four photographs of such patterns and also discusses the theory of the reflection and refraction of light by twinning layers in calcite. The large difference between the ordinary and extra-ordinary dispersive powers results in a linear source observed through such a layer appearing drawn out into spectra chanelled by interferences. Interference bands also fringe the sharply defined boundaries seen separating the different parts of the optical field of reflection and refraction.

• Some illustrations of the optical behaviour of iridescent calcite

The paper reproduces illustrations of various optical effects displayed by twinning layers in iridescent calcite. Brief descriptive notes are appended.

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