State the Stefan-Boltzmann law and explain emissivity.

The Stefan-Boltzmann law states that the radiant heat emitted by a blackbody is proportional to the fourth power of its absolute temperature: Q = σAT⁴, where σ = 5.67×10⁻⁸ W/m²-K⁴. For real surfaces, emissivity (ε) is introduced: Q = εσAT⁴. Emissivity ranges from 0 to 1, representing how effectively a surface emits radiation compared to a blackbody. Polished metals have low emissivity (~0.05), while oxidized surfaces and paints have high values (~0.9).