Thermo-optical Behavior Research Articles

The dispersion and thermo-optic behaviour of vitreous silica

The refractivity and the dispersion of vitreous silica are shown to be calculable from those of quartz by considering the diminished density. The temperature variation of its refractive indices over the entire range of wavelengths is in quantitative accord with the hypothesis that the characteristic absorption frequencies in the ultra-violet fall with rise of temperature. The rate of change of these frequencies has been traced from −160° to 1000°C. and shows remarkable variations in the vicinity of the temperature at which theα-β transformation in crystalline quartz occurs.

Further studies on the temperature variation of the refractive index of crystals

The temperature coefficients of refraction of lithium fluoride, fluorspar, magnesia and quartz have been determined interferometrically for wavelengths betweenλ 2500 andλ 6000, from room temperature to 400°C. The values have been utilised, in conjunction with dispersion formulae mostly proposed by the author, to calculate the temperature dependence of the ultra-violet frequencies, using Ramachandran’s theory of thermo-optic behaviour. The theory is found to account successfully for the variation ofdn/dt with wavelength and temperature. Similar results have been obtained in the visible region alone for the three refractive indices of barite.

Thermo-optic behaviour of solids

A direct verification of the general theory of thermo-optic behaviour has been obtained by applying it to the alkali halides. Rocksalt, sylvine and potassium iodide have been considered in detail. For this purpose, new dispersion formulae embodying observed absorption frequencies have been developed for NaCl and KI. With KI, it is found that the proportionate variation χ [=-d (log v)/dt] of the first ultra-violet frequency at 2190 A.U. measured by Fesefeldt agrees with what is calculated from the dn/dt data, thus verifying the fundamental basis of the author's theory. The theory successfully explains the whole course of the variation of dn/dt with wavelength in the case of rocksalt, and in particular the positive values of dn/dt near about 2000 A.U. and its reversal in sign with increase of wavelength. The first three ultra-violet frequencies have χ's of the order of +100×10 -6 , +25×10 -6 and -30×10 -6 respectively, the last one near 500 A.U. not varying with temperature. With sylvine,dn/dt measurements in the visible and near infra-red are satisfactorily explained with similar values of χ as in rocksalt. The paper also contains a general review of the dispersion data for the alkali halides. It is shown that a dispersion formula of the Drude form is more appropriate for these salts than one of the Lorentz-Lorenz form.

Thermo-optic behaviour of solids

Thermo-optic behaviour of solids

Thermo-optic behaviour of solids

The refractive index of zinc-blende increases appreciably on heating for all wavelengths in the visible region the increase being 0·0212 and 0.0107 respectively for wavelengths 4358 A.U. and 7320 A.U. on heating from 0° to 205° C. It is shown that by applying the general theory of Part I one can satisfactorily explain both the variation ofdn/dt with wavelength and the courses of the temperature—refractive index curves from −80° to 700° C. for different wavelengths. The calculations show that the ultra-violet dispersion frequency at 2532 A.U. alters appreciably with temperature, the proportional rate of change χ=−d (log ν)/dt being 70×10−6. This is nearly ten times the corresponding rate for diamond. The rate of change becomes less and less on lowering the temperature. It is pointed out that, for diamond, χ actually vanishes at very low temperatures and this is also probably the case for zinc-blende.

Thermo-optic behaviour of solids

Thermo-optic behaviour of solids

Thermo-optic behaviour of solids

Thermo-optic behaviour of solids

Thermo-optic behaviour of solids

Thermo-optic behaviour of solids