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Physics LibreTexts

2.5: Planck's Equation

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The importance of Planck's equation in the early birth of quantum theory is well known. Its theoretical derivation is dealt with in courses on statistical mechanics. In this section I merely give the relevant equations for reference.

Planck's equation can be given in various ways, and here I present four. All will be given in terms of exitance. The radiance is the exitance divided by π.(Equation 1.15.2.). The four forms are as follows, in which I have made use of equations 1.3.1 and the expression hν=hc/λ for the energy of a single photon.

The rate of emission of energy per unit area per unit time (i.e. the exitance) per unit wavelength interval:

Mλ=C1λ5(eK1/λT1)

The rate of emission of photons per unit area per unit time per unit wavelength interval:

Nλ=C2λ4(eK1/λT1)

The rate of emission of energy per unit area per unit time (i.e. the exitance) per unit frequency interval:

Mν=C3ν3eK2ν/T1

The rate of emission of photons per unit area per unit time per unit frequency interval:

Nν=C4ν2eK2ν/T1

The constants are:

C1=2πhc2=3.7418×1016W m2(2.6.5)C2=2πc=1.8837×109m s1(2.6.6)C3=2πh/c2=4.6323×1050kg s(2.6.7)C4=2π/c2=6.9910×1017m2s2(2.6.8)K1=hc/k=1.4388×102m K(2.6.9)K2=h/k=4.7992×1011s K(2.6.10)

Symbols:

h=Planck's constantk=Boltzmann's constantc=speed of lightT=temperatureλ=wavelengthν=frequency


This page titled 2.5: Planck's Equation is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Jeremy Tatum via source content that was edited to the style and standards of the LibreTexts platform.

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