Planck's law In 1880, André-Prosper-Paul Crova published a diagram of the three-dimensional appearance of the graph of the strength of thermal radiation as a function of wavelength and temperature. He determined the spectral variable by use of prisms. He analyzed the surface through what he called "isothermal" curves, sections for a single temperature, with a spectral variable on the abscissa and a power variable on the ordinate. He put smooth curves through his experimental data points. They had one peak at a spectral value characteristic for the temperature, and fell either side of it towards the horizontal axis. Such spectral sections are
Graph of a function should be taken into account. The graph of a function on its own doesn't determine the co-domain. It is common to use both words, function and graph of a function, since even if considered the same object, they indicate viewing it from a different angle. Given a mapping formula_1, in other words a function formula_2 together with its domain formula_3 and co-domain formula_4, the graph of the mapping is the set which is a subset of formula_6. The graph of the function formula_2 is the same set formula_8, which is, in the abstract definition of a function, the same as