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Re: Thermal Waves ?



I'm reminded of second sound.

bc

Ludwik Kowalski wrote:

Montclair State University had a Ukrainian speaker this week,
Oleh Volchanskyy. The title of his presentation was "Thermal
Wave Microscopy." I did not know that it is used as a non-
destructive tool to investigate internal structure of opaque
objects, such as tiny integrated circuits. I do not know why
but my brain resists the idea that thermal energy (heat) can
be transferred as a wave.

For the purpose of this discussion I would say T (call it
what you want if you feel it should not be called temperature)
can be defined as "what a tiny fast thermometer would show"
at any given place and time. Here is the context in which the
concept of a temperature wave T(x,t) was introduced:

A flat surface absorbs light whose intensity is changing
sinusoidally with some frequency, for example, 10 Hz or
100000 Hz. Suppose the resulting temperature fluctuations at
the surface are between 19 C and 21 C. In a layer below the
surface the temperature oscillations are repeated with
delays proportional to x. The temperature wave amplitude
decreases rapidly with depth. The speed of propagation of
heat is very slow and for that reason the wavelengths, for
any given f, is very very short. That is why extremely
small details can be recognized at various depths (up to
say 50 or 500 microns, depending on frequency).[ Heat
waves generate acoustical waves which travel long distances
(say 2 or 20 mm) and are analyzed to map the structure
where sound is produced (in a spot by spot manner).]

I was not comfortable with the concept of a "temperature
wave" because, for some reason, I usually associate waves
with restoring forces (for example, due to elasticity) and
with inertia. Mechanical waves propagate in a medium
which can be modeled as a sequence of linked harmonic
oscillators. But I can not imagine a thermal harmonic
oscillator. Any comments on this? Thanks in advance.
Ludwik Kowalski

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.