Definition

Radiation is heat that travels as invisible light waves, mostly infrared at everyday temperatures, and unlike conduction and convection it needs no medium, so it can cross a vacuum, which is how the Sun's warmth reaches Earth across empty space. All objects emit radiation; hotter objects emit more and at shorter wavelengths, dark and dull surfaces absorb and emit it well, and shiny, light surfaces reflect it. A surface at absolute temperature $T$ radiates power $P = \sigma \epsilon A T^4$ (the Stefan-Boltzmann law), where $\sigma = 5.67 \times 10^{-8}\ \text{W/(m}^2\cdot\text{K}^4)$ and $\epsilon$ is emissivity; the peak wavelength follows Wien's law, $\lambda_{\max} T = 2.90 \times 10^{-3}\ \text{m}\cdot\text{K}$, and net exchange with the surroundings is $P_{\text{net}} = \sigma \epsilon A (T^4 - T_0^4)$.