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Physics
Classical Physics
Relation between spectral intensity and spectral energy density
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[QUOTE="IcedCoffee, post: 6472046, member: 649038"] [B]TL;DR Summary:[/B] How is the following relation between spectral intensity and spectral energy density derived? In Principles of Lasers by Svelto, while deriving the Planck radiation formula, equation 2.2.3 says $$I_{\nu} = \frac {c_0} {4n} \rho_\nu$$ where ##I_\nu## is the spectral intensity at some hole in the cavity wall (energy per time per area per frequency), ##c_0## is the speed of light in vacuum, ##n## is the refractive index of the medium inside the cavity, and ##\rho_\nu## is the spectral energy density inside the cavity (energy per volume per frequency). I understand that in the case of monochromatic wave propagating in one direction, ##I = \frac {c_0} {n} \rho## since multiplying both sides by ##dt## would give the amount of energy passing through a given area (perpendicular to the direction of propagation), which must originate from the volume filled with the energy density ##\rho##. However, in this case, where does the factor of ## \frac 1 4## come from? [/QUOTE]
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Physics
Classical Physics
Relation between spectral intensity and spectral energy density
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