An ideal blackbody is simply a closed cavity like object which absorbs all incident radiation and re-emits radiation (energy) that is characteristic of the system irrespective of the type of the incident radiation when in thermal equilibrium. The radiated energy is called blackbody radiation and is produced by standing wave or resonant modes of the cavity which is radiating. The amount of radiation emitted in a given frequency range is proportional to it is number of modes in that range. Classical Physics implies that all modes will have an equal chance of being produced and that the number of modes is proportional to the square of the frequency. This behavior is dubbed as Rayleigh Jeans Law which states that the spectral irradiance of electromagnetic radiation emitted by a blackbody at a fixed temperature.
