Radiation (Stefan–Boltzmann)

Radiation: heat transfer by ELECTROMAGNETIC WAVES (mostly infrared at terrestrial T). Doesn't need a medium.

Stefan-Boltzmann law: power radiated per unit area P/A = εσT⁴, with σ = 5.67×10⁻⁸ W/m²·K⁴ and ε = emissivity (0 ≤ ε ≤ 1).

Black body: ε = 1, perfect absorber and emitter. Sun is approximately a black body at ~5800 K.

Net radiative exchange: P_net = εσA(T_body⁴ − T_surroundings⁴).

Wien's law: λ_max·T = 2.898 × 10⁻³ m·K. Peak wavelength of emitted radiation.

Sun's peak: λ_max ≈ 500 nm (visible). Human body (310 K): peak ~ 9.4 μm (infrared).

Radiation is how Earth gets energy from Sun (no medium between). Also how Earth loses heat to space.

Greenhouse effect: atmosphere absorbs outgoing IR, traps heat ⇒ Earth's surface stays warmer.