What is the name of the law that gives the wavelength of maximum blackbody radiation?

The law that gives the wavelength of maximum blackbody radiation is Wien’s displacement law. It states that the peak wavelength is inversely proportional to absolute temperature: λmax = b/T, where b is Wien’s displacement constant.

For wavelength-based spectral radiance, the constant is approximately 2.898 × 10⁻³ meter-kelvin. As an object gets hotter, its peak emission shifts toward shorter wavelengths. This explains why heated metal may glow dull red, then orange or yellow, and eventually appear whiter or bluer as its temperature rises.

Wilhelm Wien derived the law in 1893, before Max Planck developed the full quantum description of blackbody radiation. Planck’s law gives the complete emission curve; Wien’s law identifies where that curve peaks. A subtle but important point is that the peak depends on whether radiation is plotted per unit wavelength or per unit frequency. Those two peaks do not correspond to the same wavelength, so Wien’s law must be applied with the correct form of the spectrum. The Stefan–Boltzmann law is different: it gives total emitted power, proportional to T⁴.