Wien displacement law constant (wavelength)

Wien's displacement law says the peak of a blackbody's spectrum shifts inversely with temperature: λ_max = b/T. With b ≈ 2898 μm·K, the Sun's 5772 K photosphere peaks near 502 nm — green, in the middle of human vision — while a 300 K room peaks around 9.7 μm in the thermal infrared, which is exactly the band a thermal imaging camera is built to see.

Wilhelm Wien published the law in 1893, before Planck's quantum, by a clever thermodynamic argument about adiabatically compressing radiation in a mirrored cavity; he received the 1911 Nobel Prize in Physics for his work on heat radiation. The constant is the solution of a transcendental equation, b = hc/(xk) with x = 4.965114..., so it inherits exactness from h, c and k. Note that the peak depends on how you slice the spectrum — per wavelength or per frequency — and the two answers are genuinely different.