To make my question simpler, assume that there's a black body at 0K or very close to it (if we can't assume there's a body at 0K). Say I have a monochromatic source of photons which sends 1 photon on the black body. The wavelength of the photon is 500 nm so around green. If I understood well how a black body behaves, it will absorb the photon and re-emit a continuum spectra of light. In other words, it will radiate an infinity of photons. Obviously the black body can't emit another 500 nm photon otherwise it would emit only 1 photon (due to conservation of energy) and not a continuum spectra. I realize that in a realistic case the black body cannot emits an infinity of photons, but it doesn't matter for my question. How can I know what will be the spectra of a black body, knowing that I send a photon of wavelength [tex]\lambda[/tex]? So after all, the "continuum spectra" can be very limited in wavelength range? I'm interested in knowing how to determine for example what would be the most energetic photon that will be emitted in my previous example. By the way it's not a homework question so any reply of the form "look at Planck's Law" or any other Law satisfies me. Explanations are welcome too.