View source for Định luật Planck Phóng xạ vật đen

Định luật Planck cho rằng
:<math>B_\nu(T) = \frac{2h\nu^3}{c^2}\frac{1}{e^{\frac{h\nu}{kT}} - 1},</math>
Với
:B''<sub>ν</sub>''(''T'') is the spectral radiance (the [[Power (physics)|power]] per unit [[solid angle]] and per unit of area normal to the propagation) density of frequency ''ν'' radiation per unit [[frequency]] at thermal equilibrium at temperature ''T.''
:''h'' is the [[Planck constant]];
:''c'' is the [[speed of light]] in a vacuum;
:''k'' is the [[Boltzmann constant]];
:''ν'' is the [[frequency]] of the electromagnetic radiation;
:''T'' is the absolute [[temperature]] of the body.
For a black body surface the spectral radiance density (defined per unit of area normal to the propagation) is independent of the angle <math>\theta</math> of emission with respect to the normal. However, this means that, following [[Lambert's cosine law]], <math> B_\nu(T) \cos \theta</math> is the radiance density per unit area of emitting surface as the surface area involved in generating the radiance is increased by a factor <math> 1/\cos \theta</math> with respect to an area normal to the propagation direction. At oblique angles, the solid angle spans involved do get smaller, resulting in lower aggregate intensities.
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