1. What is a Blackbody?
A blackbody is an ideal physical model under the condition of energy conservation (thermal equilibrium) that can completely absorb all incident electromagnetic radiation energy at all wavelengths, without producing any reflection or transmission.

According to the law of conservation of energy:
For transparent objects: Incident radiant energy = Absorption + Reflection + Transmission
For opaque objects: Incident radiant energy = Absorption + Reflection
When the reflectance of an opaque object is 0, its absorptive capacity reaches its maximum, and its corresponding thermal radiation capacity is also at its maximum.

An absolute blackbody is an ideal model that completely converts absorbed energy into thermal radiation for release. Its core characteristic is that the spectral power distribution of its radiation depends solely on the absolute temperature (unit: Kelvin, K), and is independent of the material's chemical composition and geometric shape.
In the CIE 1931 xy chromaticity diagram (see Figure 1), the chromaticity coordinates of a blackbody at different temperatures form a continuous curve known as the Planckian locus. As the temperature increases, the dominant hue of the radiation shifts from warm red (low frequency, long wavelength) to cool blue (high frequency, short wavelength). This locus is an important basis for defining correlated color temperature (CCT) and is widely used in light sources, imaging systems, and color calibration.

Figure 1[1]: CIE 1931 xy chromaticity diagram and Planckian locus

2. Blackbody Simulation Model
In engineering and experiments, an isothermal cavity with a small aperture is commonly used as an approximate blackbody model (see Figure 2). After incident electromagnetic waves enter the small aperture, they undergo multiple reflections inside the cavity and are fully absorbed, making it difficult for them to escape again. This achieves a reflectance close to 0, allowing it to be approximated as an ideal blackbody.

Figure 2[2]: Blackbody simulation model

3. Radiation Laws of Blackbody
The thermal radiation characteristics of a blackbody strictly follow the following three major laws:
(1) Planck's Law
Describes the relationship between the spectral radiant exitance of a blackbody, wavelength, and temperature. The core formula is:
$$M_\lambda(\lambda, T) = \frac{c_1}{\lambda^5} \frac{1}{e^{\frac{c_2}{\lambda T}} - 1}$$ where:
$M_\lambda$: spectral radiant exitance ($W \cdot m^{-3}$);
$\lambda$: wavelength (m);
$T$: temperature (K);
$c_1 = 2\pi h c^2$: first radiation constant ($W \cdot m^2$);
$c_2 = \frac{hc}{k_B}$: second radiation constant ($m \cdot K$);
$k_B $: Boltzmann constant ($J \cdot K^{-1}$).

Figure 3[3]: Schematic diagram of Planck's law

As shown in Figure 3, as the blackbody temperature increases (e.g., from 100K to 10000K), the peak wavelength of radiation shifts towards the short-wavelength direction (corresponding to Wien's displacement law); meanwhile, the total radiant energy across all wavelengths of the blackbody increases sharply with temperature at a rate of $T_{4}$ (corresponding to the Stefan-Boltzmann law).

(2) Wien's Displacement Law
Reveals the inverse relationship between the peak wavelength $\lambda_m$ of blackbody radiation and the absolute temperature $T$. The core formula is:
$$\lambda_m T = b$$ where:
$\lambda_m$: peak wavelength (m)
$T$: temperature (K)
$b$: Wien's displacement constant ($m \cdot K$)
$b \approx 2.897771955 \times 10^{-3} \, m \cdot K$
This law explains that as the temperature of a blackbody increases, the peak wavelength of its emission spectrum shifts towards the short-wavelength direction.

(3) Stefan-Boltzmann Law
Describes the relationship between the total radiant exitance $M$ of a blackbody across all wavelengths and the fourth power of its absolute temperature $T$. The core formula is:
$$M(T) = \int_0^\infty M_\lambda(\lambda, T) d\lambda = \frac{c_1 \pi^4}{15 c_2^4} T^4 = \sigma T^4$$ where:
$M $: radiant exitance ($W \cdot m^{-2}$)
$T$: temperature (K)
$c_1 = 2\pi h c^2$: first radiation constant ($W \cdot m^2$)
$c_2 = \frac{hc}{k_B} $: second radiation constant ($m \cdot K$)
$\sigma$: Stefan-Boltzmann constant ($W \cdot m^{-2} \cdot K^{-4}$)

Figure 4[4]: Relationship between total radiant exitance of a blackbody and temperature

This law proves the high sensitivity of radiation capacity to temperature: a slight increase in temperature leads to a dramatic increase in total radiant energy, quantitatively supporting the conclusion that “an ideal absorber is the strongest emitter.”

Image sources:
[1] https://upload.wikimedia.org/wikipedia/commons/thumb/b/ba/PlanckianLocus.png/960px-PlanckianLocus.png
[2]https://en.wikipedia.org/wiki/Black_body#/media/File:Black_body_realization.svg
[3] https://commons.wikimedia.org/wiki/File:BlackbodySpectrum_loglog_en.svg
[4]https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law#/media/File:Stefan_Boltzmann_001.svg