Blackbody radiation
Every object with temperature glows. Planck’s law describes the spectrum of that glow — and its discovery in 1900 launched quantum mechanics. Slide the temperature and watch the spectral curve shift, the color change, and the total power grow as T4.
B(λ, T) = (2hc² / λ&sup5;) · 1 / (ehc/λkBT − 1)
Planck’s law
In 1900, Max Planck derived the correct formula for blackbody radiation by assuming that energy is emitted in discrete quanta: E = hν. This was the birth of quantum mechanics. The formula gives the spectral radiance — power per unit area, per unit solid angle, per unit wavelength — as a function of wavelength and temperature.
Wien’s displacement law
The peak wavelength is inversely proportional to temperature: λmax = b/T, where b ≈ 2.898 × 10−3 m·K. This is why hot stars appear blue (short wavelength peak) and cool stars appear red (long wavelength peak).
The ultraviolet catastrophe
Before Planck, the Rayleigh-Jeans law predicted that spectral radiance would increase without bound at short wavelengths — the ultraviolet catastrophe. Toggle the classical prediction overlay to see how it diverges from reality, and why classical physics was forced to yield to quantum mechanics.
Stefan-Boltzmann law
The total power radiated per unit area is proportional to T4: P = σT4, where σ ≈ 5.67 × 10−8 W·m−2·K−4. Doubling the temperature increases the total radiated power by a factor of 16.