Blackbody radiation

The model is defined by what it absorbs. A blackbody takes in all radiation that falls on it, and if it sits in equilibrium at constant temperature it must in turn emit energy (15.4.A.2). 15.4.A.3 makes that emitted spectrum continuous and dependent on temperature alone, usually plotted as intensity per unit wavelength against wavelength.

Two sheet equations describe the curve, and both are required content.

• Wien's law, $\lambda_{\max} = b/T$, locates the peak, with $b = 2.90 \times 10^{-3} \, \text{m} \cdot \text{K}$. A hotter body peaks at a shorter wavelength.
• The Stefan-Boltzmann law, $P = A \sigma T^4$, gives total emitted power, with $\sigma = 5.67 \times 10^{-8} \, \text{W}/(\text{m}^2 \cdot \text{K}^4)$.

$T$ is in kelvin in both, and the fourth power is what makes the second one bite: halving the peak wavelength doubles the temperature and raises the power by a factor of sixteen.

The quantum point is 15.4.A.3.i. The intensity distribution cannot be modeled with classical physics alone, and is described by Planck's law, which assumes that the energy of light is quantized. Planck's law itself is printed neither in the CED nor on the sheet, so name it and stop. The phrase ultraviolet catastrophe appears in the CED only as an essential question on the unit opener.