I have read the derivation of the Rayleigh-Jeans law from: http://www.cv.nrao.edu/course/astr534/BlackBodyRad.html This derivation is quite similar to the derivation in my textbook. My question is why the frequency/wavelength is quantized, but there still a d[itex]\nu[/itex] or d[itex]\lambda[/itex]. Not this "d" only apply to continuous variables? And since the frequency is quantized, that means the emission spectrum is not continuous? But my textbook said "condensed state emits a continuous spectrum of radiation."