Blackbody

A blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, across all wavelengths and directions. It emits radiation in a characteristic spectrum that depends only on its temperature, not on its shape or composition. In practice, no real object is a perfect blackbody, but many systems approximate this behavior closely enough for experimental and theoretical use.

Planck's law describes the spectral radiance of a blackbody. The spectral radiance per unit wavelength is Bλ(T) = (2hc^2)/λ^5 · 1/(exp(hc/(λkT)) − 1), where h is Planck's constant, c is the speed of light, k is Boltzmann's constant, λ is wavelength, and T is absolute temperature. The law implies a continuous spectrum whose shape shifts with temperature.

A perfect blackbody has emissivity ε = 1, meaning it is an ideal absorber and emitter. Real objects have emissivities less than 1 and thus are only approximate blackbodies. A common practical realization is a cavity with a small, light-absorbing hole; radiation entering the cavity is effectively absorbed, and the hole's emission approximates a blackbody spectrum.

The total power emitted per unit area of a blackbody is given by the Stefan-Boltzmann law: P/A = σT^4, where σ ≈ 5.670374×10−8 W m−2 K−4. The spectral distribution peaks at a wavelength given by Wien's displacement law, λmax = b/T with b ≈ 2.8977719×10−3 m·K.

Historically, the concept arose from Kirchhoff's law of thermal radiation, and Planck's introduction of quantum theory in 1900 provided the correct blackbody spectrum. This work established quantum mechanics and the quantization of energy.

Applications include calibrating radiation detectors, radiometry, astrophysics, and climate science, where blackbody-like spectra serve as reference models.