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A blackbody is an object that absorbs all light incident on it, regardless of color. Consequently, such an object reflects no light at all, and so it appears perfectly black. However, a good absorber is also a good emitter, which means that blackbodies are emitters of light. We don’t notice the light from room-temperature blackbodies because it is almost all infrared radiation, which our eyes cannot see. However, light from hotter blackbodies is visible. The red glow of hot coals in a fireplace is a common example, corresponding to a temperature of about 1,000 K, and the solar spectrum is a good approximation to a 6,000 K blackbody.
The spectrum of blackbody radiation is described mathematically by Planck’s radiation law, which depends on only one parameter: temperature. However, calculations with Planck’s law are tedious. iBlackbody allows you to explore blackbody calculations easily in order to gain a better understanding of blackbodies.