Page 10 - 83 basic knowledge of astronomy
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s
dΩ
θ
dσ
Figure 8: Definition of Specific/Monochromatic Intensity I ν (or Brightness
B ν ).
7.2 Constancy of the Monochromatic Intensity I ν
The monochromatic intensity I ν remains constant along a ray path, irre-
spective of the distance from the emitting source, as long as no absorption,
emission, or scattering occurs along the path. An explanation of this con-
stancy may be given as follows.
W2
σ 2
Ω 2
W 1
σ
1
r 2
Ω
1
r 1
Figure 9: Constancy of the monochromatic intensity I ν along the ray path.
Let us assume a spherically symmetric distribution of radiation from an
emitting source at the center, without any intervening absorption, emission,
or scattering, as shown in Figure 9. Let a cone-like ray tube of constant
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