Abstract
MANY years ago1, it was pointed out by one of us that Debye's concept identifying the thermal energy of a solid with the energy of elastic vibrations within it having a wide range of frequencies has an important optical consequence, namely, that a beam of light traversing a transparent solid would be scattered to an extent depending upon the energy of thermal agitation. This conclusion was also verified experimentally in a semi-quantitative fashion2. The principal experimental difficulty in studying the subject was that of obtaining crystals sufficiently large and at the same time free from imperfections or inclusions, these conditions being necessary to prevent the feeble thermal opalescence being overpowered by parasitic diffuse light. The same difficulty appears in attempting to investigate the thermal opalescence in crystals by spectroscopic methods. The elastic waves, longitudinal or transverse as the case may be, which scatter the light being progressive, they should give rise to Brillouin-Doppler shifts of optical frequency corresponding to their respective acoustic velocities. If parasitic light be present, the unmodified scattering and the hyper-fine structure components usually accompanying the same would overpower the Brillouin-Doppler components to be expected.
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