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Every heated body emits radiation. However, classical physics could not explain the observed emission spectrum for a theoretical model called "blackbody".

A blackbody is an ideal object in thermodynamic equilibrium with its environment which behaves like a perfect "radiation absorber" (all incident radiation is absorbed and there is neither reflection nor transmission, which explains why such a body would appear to us as black). In order to remain in thermodynamic equilibrium, the blackbody thus heated emits electromagnetic radiation in all wavelengths. The energy radiated per unit of time and area, called luminance, or power per unit of area, covers all wavelengths and depends only on the surface temperature of the body, as illustrated by the curves of the animation.

It is Max Planck who, in 1900, formulated his famous equation which describes the law that now bears his name: Planck's law.

His work validates the emerging theories of quantum mechanics since, to explain the macroscopic behavior of a blackbody, he considers that the atoms of the blackbody interact with electromagnetic radiation in a discrete (non-continuous) manner. Light energy is absorbed (and emitted) in the form of energy packets (the "quanta"). The seemingly continuous luminance is actually the statistical result of the sum of the individual quantum effects.

Planck's law has many consequences. Among them is the fact that the shape of the curve depends only on the temperature of the heated body and not on the material of which it is made. Thus, the Sun radiates in the same way as a piece of metal heated to the same temperature (5800° K).

Let us also quote Wien’s law which states that the maximum wavelength λMax is inversely proportional to the temperature:

λ_{Max} ∝ 1 / T

Boltzmann's law which states that the total radiated power (per unit area) is proportional to T4

M = σ.T^{4}

- To illustrate the theoretical curve of Planck's law.
- To illustrate how temperature modifies theoretical curves.
- To explain that the blackbody curve at 3° Kelvin (fossil radiation) is a testimony of the Big-Bang.

**Fossil radiation**.

Planck’s law on blackbody radiation appears in cosmology with the discovery of the cosmic microwave background. Indeed, the **Big Bang** cosmological model was slow to take hold…