Introduction

Radiation heat transfer plays a fundamental role in thermodynamics and engineering, with the concept of a black body serving as a cornerstone in understanding how objects absorb and emit thermal radiation. A black body is an idealized object that absorbs all incident radiation, making it a perfect reference for studying real-world materials and their radiative properties. Its behavior is crucial for various scientific and technological applications, from astrophysics to thermal engineering.

In this post, we explore the defining characteristics of a black body, its absorptivity and emissivity, and the principles governing bodies in thermodynamic equilibrium, including Kirchhoff’s Law. Understanding these concepts allows us to analyze how real objects interact with thermal radiation and how energy exchange is regulated in different systems.

Definition of the Black Body

A black body is an object that allows all incident radiation to enter into its interior and absorbs it completely (no radiation is reflected at the external surface or exits the body after entering into it), regardless of wavelength and angle of incidence. As such, a black body is a perfect absorber for all incident radiation [1].

Furthermore, a black body also diffusely (equally in all angles) emits the maximum possible radiant energy at each wavelength for any body at a particular temperature \(T\) [1]. This maximum possible radiant energy at each wavelength and temperature is given by Planck’s Law [2].

Absorptivity and Emissivity

Absorptivity

The absorptivity \(\alpha(\lambda)\) of a general body is a unitless coefficient defined as the ratio of the power the body absorbs at a specific wavelength to the total incident power.

\[ \alpha(\lambda) = \frac{\text{power absorbed by the body at } \lambda}{\text{total incident power at } \lambda} \]

Since the black body is a perfect absorber, it follows that \(\alpha_\text{blackbody} = 1\), \(\forall \lambda\). In this way, the power absorbed by a black body can be used as a reference to obtain the absorptivity of other bodies.

\[ \alpha(\lambda) = \frac{\text{power absorbed by the body at } \lambda}{\text{absorbed power at } \lambda \text{ if the body was a black body}} \]

Emissivity

The emissivity \(\varepsilon(\lambda)\) of a general body is a unitless coefficient defined as the ratio of the power emitted by the body at a specific wavelength to the maximum possible power emitted by any body at the same temperature as the general body.

\[ \varepsilon(\lambda) = \frac{\text{power emitted by the body at } \lambda}{\text{maximum power emitted by any body at the same temperature at } \lambda} \]

Since the black body is a perfect emitter, it follows that \(\varepsilon_\text{blackbody} = 1\), \(\forall \lambda\), since it emits the maximum radiant energy that any body can emit for a particular temperature. In this way, the power emitted by a black body can be used as a reference to obtain the emissivity of other bodies.

\[ \varepsilon(\lambda) = \frac{\text{power emitted by the body at } \lambda}{\text{power emitted at }\lambda\text{ by a black body at the same temperature}} \]

Thermodynamic Equilibrium

Bodies in Thermodynamic Equilibrium and Kirchhoff’s Law

A body is in thermodynamic equilibrium with its surroundings when it emits the same amount of energy that it absorbs. Thus, there is no net energy exchange between the body and its surroundings, and the body’s temperature remains constant.

One consequence of thermodynamic equilibrium is Kirchhoff’s Law of Thermal Radiation [3] which, simply put, states that, for an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, its absorptivity function \(\alpha (\lambda)\) and its emissivity function \(\varepsilon (\lambda)\) are the equal. In other words, the absorptivity and emissivity of a body in thermal equilibrium are equal at each wavelength, \(\alpha (\lambda) = \varepsilon (\lambda) \).

Note that the inverse is not necessarily true: a body such that \(\alpha (\lambda) = \varepsilon (\lambda) \), \(\forall \lambda\), is not necessarily in thermodynamic equilibrium. A clear example of this is the black body, which always satisfies \(\alpha (\lambda) = \varepsilon (\lambda)\) (equal to \(1\)) but is not necessarily in thermodynamic equilibrium with its surroundings.

Note also that \(\alpha (\lambda) = \varepsilon (\lambda) \) does not mean that in thermodynamic equilibrium the power absorbed by the body at each wavelength is equal to the power emitted by the body at that same wavelength, but rather that the absorptivity and emissivity functions are equal at each wavelength. In the case of the absorptivity, we are comparing the power absorbed by the body to the total incident power, while in the case of the emissivity, we are comparing the power emitted by the body to the maximum possible power emitted by any body at the same temperature, which is given by Plank’s Law:

\[ \text{power absorbed by the body at } \lambda = \alpha (\lambda) \times \text{total incident power at } \lambda \] \[ \text{power emitted by the body at } \lambda = \varepsilon (\lambda) \times \genfrac{}{}{0pt}{}{\text{power emitted at }\lambda \text{ by a black body}}{\text{at the same temperature}} \]

Clearly, these two quantities are not necessarily equal, even if \(\alpha (\lambda) = \varepsilon (\lambda)\). In fact, the power emitted by the body at a specific wavelength does not depend at all on the distribution of the absorbed power in wavelength, but only on the temperature of the body and the absorptivity (or emissivity, since we are in thermodynamic equilibrium) functions at that wavelength.

What does need to be equal, however, is the total power absorbed by the body and the total power emitted by the body, which is a consequence of the body being in thermodynamic equilibrium with its surroundings. This is obtained by integrating the power absorbed and emitted over all wavelengths:

\[ P_\text{absorbed} = \int_0^\infty \alpha (\lambda) \times \text{total incident power at } \lambda \, d\lambda \]

\[ P_\text{emitted} = \int_0^\infty \varepsilon (\lambda) \times \genfrac{}{}{0pt}{}{\text{power emitted at }\lambda \text{ by a black body}}{\text{at the same temperature}} \, d\lambda
\]

Thermodynamic equilibrium implies \( P_\text{absorbed} = P_\text{emitted} \).

Conclusion

A black body is defined as a perfect absorber and emitter of radiation, making it a fundamental reference in radiation heat transfer. Its properties allow us to define absorptivity and emissivity, which describe how real materials interact with thermal radiation. Kirchhoff’s Law establishes that for a body in thermodynamic equilibrium, absorptivity and emissivity are equal at each wavelength.

A common misconception is that equal absorptivity and emissivity at each wavelength implies that the power absorbed and emitted at each wavelength must also be equal. However, this is not the case—absorptivity relates to the fraction of incident radiation absorbed, while emissivity compares emitted power to that of an ideal black body at the same temperature. The true condition for equilibrium is that the total power absorbed equals the total power emitted, which results from integration over all wavelengths.

Another frequent misunderstanding is that a black body is always in thermodynamic equilibrium. While a black body satisfies \(\alpha(\lambda) = \varepsilon(\lambda) = 1\) for all wavelengths, it does not necessarily mean it is in equilibrium with its surroundings. Instead, equilibrium depends on the balance between total absorption and emission, rather than the individual values of \(\alpha(\lambda)\) and \(\varepsilon(\lambda)\).

By carefully distinguishing these principles, we can avoid misinterpretations and correctly apply black body concepts in thermal radiation problems.

Bibliography

[1] Siegel, R., & Howell, J. R. (2002). Thermal radiation heat transfer; Volume 1 (4th ed.). Taylor & Francis.

[2] Planck, M. (1901). On the law of distribution of energy in the normal spectrum. Annalen der Physik, 4(3), 553–563.

[3] Kirchhoff, G. (1860). On the relation between the radiating and absorbing powers of different bodies for light and heat. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 20(130), 1–21. https://doi.org/10.1080/14786446008642901