What is a blackbody? How do real bodies differ from blackbodies?

A blackbody is an idealized object that absorbs all incoming electromagnetic radiation, regardless of its wavelength or direction. It also emits radiation with a distinct spectrum, known as the blackbody spectrum, which depends solely on its temperature. In other words, blackbodies are perfect absorbers and perfect emitters of radiation.

When a blackbody is heated, it emits radiation across a range of wavelengths. The amount of energy emitted at each wavelength is given by Planck's radiation law. The peak of the blackbody spectrum, as well as the total energy emitted, shift to shorter wavelengths and higher intensities as temperature increases. This relationship is described by Wien's displacement law and the Stefan-Boltzmann law, respectively.

The best real-world approximation of a blackbody is graphite. Various real-world materials have emissivities close to one but not exactly equal to one. Graphite, which has an emissivity of about 0.99, is considered to be the closest natural material to a blackbody due to its high absorptivity and emissivity.

Real bodies differ from blackbodies in terms of their absorptivity and emissivity. In general, real bodies do not absorb all incoming radiation and do not emit radiation solely based on their temperature. A real body's absorptivity and emissivity depend on its material properties, surface condition, and the wavelength and direction of incoming radiation.

Real bodies can also be more selective in the wavelengths they absorb and emit compared to blackbodies, which makes them deviate from the blackbody spectrum. Greenhouse gases, for example, selectively absorb and emit infrared radiation, causing what is known as the greenhouse effect. Similarly, certain materials exhibit strong absorption and emission characteristics in specific wavelength ranges, making them useful in applications such as solar thermal energy conversion and stealth technology. In summary, blackbodies are idealized objects that absorb and emit radiation perfectly based on their temperature, while real bodies have more complex behaviors in terms of their absorptivity and emissivity based on their material properties, surface condition, and the direction of incoming radiation.