Planck spectrum B(λ,T)
Peak λ (Wien's law)
Every hot object glows, and the color and spectrum of that glow depend on temperature alone — this is blackbody radiation, one of the foundational puzzles that gave birth to quantum physics. This 2D companion plots the Planck spectral-radiance curve B(λ,T) = 2hc²/[λ⁵(e^(hc/λkT) − 1)] live as you drag the temperature slider from a cool 1,000 K ember to a 15,000 K blue-white star, while a glowing disc beside the chart shifts color to match. A moving marker tracks the peak wavelength predicted by Wien's displacement law (λ_peak·T = 2.8977719×10⁻³ m·K), and readouts show the total radiated power scaling as T⁴ per the Stefan-Boltzmann law and the fraction of that light falling in the visible band. Trying to explain this exact curve with classical physics produced the "ultraviolet catastrophe"; resolving it in 1900 required Max Planck to propose that radiation is emitted in discrete energy quanta, launching quantum theory — the same physics explored in the site's interactive 3D version of this demonstration.