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Unformatted text preview: 3.3.2 Specic intensity I
Occasionally one wants to do the energy bookkeeping in terms of wavelength
instead of frequency . On then denes I by the relation jI d j = jI dj
So that dE = I d2 dt d d
. (The absolute value bars are to be sure that
energy comes out positive.) Using = c=,
d = c = 2 :
d 2 c
So
2
c
I = c I = 2 I : Do not be confused by these extra factors of or . They are just bookkeeping to make \per frequency" and \per wavelength" come out consistently
when describing the same amount of energy. 3.3.3 Net ux
If we want to know the total energy deposited onto a specic element of area
dA (on a detector, say), we integrate up the specic intensity coming from
solid angle elements in all directions. However, they do not all get equal
weight: the farther they are from normal incidence, the less \projected area"
of detector is available to them.
dΩ θ
normal direction
dA 33 ...
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 Fall '09
 dion

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