When Heat Becomes Light
Every hot object glows. Heat a piece of iron and it shifts from invisible infrared to dull red, bright orange, yellow-white, and eventually blue-white. This progression follows Planck's law of black body radiation — one of the foundational equations of quantum mechanics. Max Planck derived it in 1900 by proposing that energy is quantized, inadvertently launching the quantum revolution. This simulation lets you see the black body spectrum at any temperature and watch how it maps to perceived color.
Wien's Displacement Law
Wien's law provides an elegant shortcut: the peak wavelength of emission is inversely proportional to temperature. At 3000K (incandescent bulb), the peak is at 966nm — deep in the infrared, with only the tail of the distribution in the visible range, producing warm yellow light. At 5800K (the sun), the peak is at 500nm, right in the green part of the visible spectrum. At 10000K, the peak shifts to 290nm in the ultraviolet, and the visible emission skews blue.
Color Temperature in Daily Life
The concept of color temperature is everywhere in modern life. Light bulb packaging lists it in Kelvin: 2700K for warm white, 4000K for neutral, 6500K for daylight. Camera white balance settings use color temperature to correct for ambient lighting. Display calibration targets 6500K (D65) as the standard white point. Your phone's "night mode" reduces color temperature in the evening, shifting from blue-white to amber to reduce melatonin suppression and improve sleep quality.
Beyond Ideal Black Bodies
Real light sources deviate from perfect black body spectra. Fluorescent lights have spiky emission from mercury vapor and phosphors. LEDs combine a blue pump with yellow phosphor, creating a two-peaked spectrum. These deviations are quantified by correlated color temperature (CCT) — the black body temperature whose perceived color most closely matches the light source — and by Duv, which measures distance from the Planckian locus in chromaticity space. Together, CCT and Duv fully characterize a light source's color appearance.