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In physics, a black body is an object that absorbs all electromagnetic radiation, being purely non-reflective and opaque. Accordingly, its color is dictated only by its temperature. Different temperatures bounce atoms around at different intensities which correspond to respective wavelengths of electromagnetic radiation being produced. Black bodies and issues surrounding their radiation are especially famous for their role in the formulation of quantum mechanics in the early 20th century.
Black-body radiation is sometimes also called cavity radiation, because in a laboratory, the closest approximation to a black body is a small hole connected to a larger cavity. Because any incoming light has to bounce around the interior of the cavity multiple times for it to be reflected back out, during which it is almost certain to be absorbed, the cavity hole nicely approximates the criteria of non-reflectiveness for black bodies. According to Gustav Kirchhoff, the physicist who introduced the terms "black body" and "black-body radiation" in 1860, the spectrum emanated from the whole will depend only on the temperature of the cavity and not at all on the particular materials being heated.
As the temperature of a black body increases, it emits electromagnetic radiation at higher intensities and shorter wavelengths. Around 1000 K (Kelvin, the same as Celsius but 0 is absolute zero, –273.15 °C), black-body radiation is red, from 2000 K to 4000 K, the radiation is orange, then begins to turn white at temperatures past 4000 K, at which all typical substances are in a liquid form.
In the real world, the closest approximation to black-body radiation is the cosmic microwave background, the "echo" of the Big Bang. Black holes can be described as black bodies, and it was Stephen Hawking who discovered they emit their own black-body radiation - which was named Hawking radiation in his honor.
Attempts to characterize the emissions spectrum of black-body radiation is what led scientists such as Planck and Einstein to suggest that electromagnetic radiation is quantized, which eventually led to the quantum mechanics revolution.