Temperature measurement
In fluid flow and temperature measurements, changes in the phase of a light beam originate
from the variation in the refractive index of the medium itself. The refractive index of a
transparent material is defined as
Where  is the speed of light. The value of  is greater than or equal to unity. The reduction
in the speed of light in matter can also be viewed as an increase in the optical path length
to be covered by the electromagnetic waves and hence a source of phase difference. The
density and refractive index of a transparent material are uniquely related, and to a
leading order, they are connected by the Lorenz-Lorentz formula
In gases  and the relationship reduces to
Hence, in gases, the derivatives
In liquids, the derivative is nearly constant if the bulk changes in density are small.
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