ay45c3-page9 - 3.3.2 Speci c intensity I Occasionally one...

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Unformatted text preview: 3.3.2 Speci c intensity I Occasionally one wants to do the energy bookkeeping in terms of wavelength  instead of frequency  . On then de nes 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 speci c element of area dA (on a detector, say), we integrate up the speci c 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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