Rayleigh-Jeans law

<math> f(\lambda) = 8\pi k\frac{T}{\lambda^4}</math>

where λ is in meters, T is the temperature in Kelvins, and k is Boltzmann's constant.

The law agrees with experimental measurements for long wavelengths but disagrees for short wavelengths.

Max Planck revised the law, which states:

<math>f(\lambda) = 8\pi hc \frac{\lambda^{-5}}{e^\frac{hc}{\lambda kT}-1}</math>

where h is Planck's constant, c is the speed of light.