When energized by some energy source, discrete atoms and molecules tend to emit energy at specific frequencies. These are called line spectra (atoms) or band spectra (molecules).

When energized, condensed objects give off radiation that is characteristic of their temperatures. A blackbody radiator is an ideal object that absorbs all and reflects none of the radiant energy falling on it. A study of blackbody radiation[local] led Max Planck to develop the quantum theory.

Without offering an explanation for this, Planck suggested that electromagnetic energy was quantized. Radiation consists not of a continuum of some sort, but of packages or particles called quanta. This was expressed in the fundamental relationship, E = hν.

At this moment, as you are reading this material, you are emitting radiation. That radiation is characteristic of an object heated to a temperature of 310 K (98.6 °F). This radiation is centered in the infrared region of the electromagnetic spectrum.

As objects are heated to higher temperatures, they give off more and more radiation. Also, the freqeuncy (color) shifts. So, as an object is heated from room temperature, it begins to emit what seems to be dull red light around 500 °C. As it is heated more and more, the color shifts to red, then orange, then yellow, and eventually white heat. See the supplement on blackbody radiation for Wien's law and the Stefan-Boltzmann law.