# Why there are three equations governing blackbody emission?

I learnt that the spectrum of a black body can be explained using

1. Planck distribution
2. Rayleigh -Jeans equation and
3. Weins displacement law

Can anyone tell me why there are three different equations that explain the same physical phenomena and when should I consider which equation?

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Wien's law is not accurate! It's only approximately close to the Planck's law for shorter wavelengths! Hence the term approximation. Also, Wien's displacement law is different from the other three, as in it only describes the relation between the peak wavelength and the temperature of the blackbody. $\lambda_{max} T = const$ (en.wikipedia.org/wiki/Wien's_displacement_law) The other 3 laws talk about the spectrum of a black body, i.e. the intensity as a function of wavelength/frequency, and Wien's law is one of them. (en.wikipedia.org/wiki/Wien_approximation) –  Takku Jul 5 '14 at 15:01