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All bodies above 0 K emit thermal radiation (electromagnetic waves). Stefan-Boltzmann law: $P = e\sigma AT^4$ for a body of emissivity $e$ (0 to 1), area $A$, at temperature $T$. The constant $\sigma = 5.67 \times 10^{-8}$ W m$^{-2}$ K$^{-4}$. Net radiation in surroundings at $T_s$: $P_{\text{net}} = e\sigma A(T^4 - T_s^4)$.

Wien's displacement law: $\lambda_{\max} T = 2.898 \times 10^{-3}$ m K — hotter objects emit at shorter wavelengths. The Sun (5800 K) peaks at 500 nm (visible); human body (310 K) peaks at 9.4 micrometers (infrared).

Kirchhoff's law: at thermal equilibrium, emissivity = absorptivity ($e = a$). Good absorbers are good emitters. A blackbody ($e = 1$) is the ideal case.

Key radiation scaling: $P \propto T^4$ means small temperature increases cause large power changes. Doubling absolute temperature gives 16x radiation. Critical trap in JEE: always convert to Kelvin before using radiation formulas — 27 degrees C = 300 K, not 27 K.

Cooling rate of a sphere: $dT/dt \propto 1/r$ — smaller bodies cool faster (higher surface-to-volume ratio).