A black cavity absorbs all the radiation that enters. A black body is a cavity radiator. When hotter than the surroundings it emits as an ideal radiator with a continuous emission spectrum (right). The peak emission wavelength shifts to shorter wavelengths as the temperature rises.

At 3 K the peak is in the microwave region; at 300 K in the infrared; at 6,000 K in the visible (green); at 2,000,000 K the peak is in the soft X-ray region.