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/λT−1)
The rate of emission of photons per unit area per unit time per unit wavelength interval:
Nλ=C2λ4(eK1/λT−1)
The rate of emission of energy per unit area per unit time (i.e. the exitance) per unit frequency interval:
Mν=C3ν3eK2ν/T−1
The rate of emission of photons per unit area per unit time per unit frequency interval:
Nν=C4ν2eK2ν/T−1
The constants are:
C1=2πhc2=3.7418×10−16W m2(2.6.5)C2=2πc=1.8837×109m s−1(2.6.6)C3=2πh/c2=4.6323×10−50kg s(2.6.7)C4=2π/c2=6.9910×10−17m−2s2(2.6.8)K1=hc/k=1.4388×10−2m K(2.6.9)K2=h/k=4.7992×10−11s K(2.6.10)
Symbols:
h=Planck's constantk=Boltzmann's constantc=speed of lightT=temperatureλ=wavelengthν=frequency