I am having some issues with the derivation of the Planck Radiation Formula, as for instance given in http://hyperphysics.phy-astr.gsu.edu/hbase/mod6.html . My point is that the assumption of cavity modes implies the formation of standing waves inside the cavity walls. In most situations in nature (e.g. the sun) it is however absolutely inconceivable that here standing waves could develop, simply because there are no walls or other sharp boundaries. So with the fundamental theoretical assumption in the derivation not applicable at all, why is it that the Planck Radiation Formula still describes the solar spectrum to a good approximation?