Kamis, 24 Januari 2013
Theory of critical opalescence
Einstein returned to the problem of thermodynamic fluctuations, giving a
treatment of the density variations in a fluid at its critical point.
Ordinarily the density fluctuations are controlled by the second
derivative of the free energy with respect to the density. At the
critical point, this derivative is zero, leading to large fluctuations.
The effect of density fluctuations is that light of all wavelengths is
scattered, making the fluid look milky white. Einstein relates this to Raleigh scattering, which is what happens when the fluctuation size is much smaller than the wavelength, and which explains why the sky is blue.[96]
Einstein quantitatively derived critical opalescence from a treatment
of density fluctuations, and demonstrated how both the effect and
Rayleigh scattering originate from the atomistic constitution of matter.
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