handbook of modern sensors

3.12 Heat Transfer 103
Fig. 3.41. Spectrum of electromagnetic radiation.
becomes insignificant. In these cases, thermal conductivity of air and radiative heat
transfer should be considered instead.
3.12.3 Thermal Radiation
It was mentioned earlier that in any object, every atom and every molecule vibrate.
The average kinetic energy of vibrating particles is represented by the absolute temperature.According
to laws of electrodynamics, a moving electric charge is associated
with a variable electric field that produces an alternating magnetic field. In turn, when
the magnetic field changes, it results in a changing electric field coupled with it and so
on. Thus, a vibrating particle is a source of an electromagnetic field which propagates
outwardly with the speed of light and is governed by the laws of optics. Electromagnetic
waves can be reflected, filtered, focused, and so forth. Figure 3.41 shows the
total electromagnetic radiation spectrum which spreads from γ -rays to radio waves.
The wavelength directly relates to frequency, ν, by means of the speed of light c
in a particular media:
λ = c ν . (3.128)
A relationship between λ and temperature is more complex and is governed by
Planck’s law, which was discovered in 1901. 17 It establishes radiant flux density
17 In 1918, Max K. E. L. Planck (Germany, Berlin University) was awarded the Nobel Prize
“in recognition of his services he rendered to the advancement of Physics by his discovery
of energy quanta.”