Blackbody radiation is the thermal radiation emitted from, rather than reflected by, an object. Physically, blackbody radiation is always mixed with reflected radiation, because all real-world objects reflect some of the electromagnetic waves they receive. Nonetheless, a hypothetical black body would emit radiation dependent only on its temperature. Hot black bodies would emit substantially in the visible range of light and, therefore, would be visible to humans. Cool black bodies would emit mainly in the infrared range, making them invisible to the human eye.
A black body is an object that absorbs all incoming electromagnetic radiation. No incident radiation is reflected by the body’s surface. Black bodies do not exist in the physical world; they are idealized objects used for conceptual and theoretical purposes. Real-world bodies always reflect some portion of incoming radiation, though this amount can vary. Black bodies are named as such because, having reflected no incident light waves, they would appear black when cold.
All objects, including black bodies, emit an amount of thermal radiation that is dependent on the object’s temperature. In fact, this amount is proportional to an object’s absolute temperature to the fourth power. Doubling an object’s absolute temperature—for example, turning an oven from room temperature to about 600° Fahrenheit (315° Celsius)—will result in 16 times as much thermal radiation. The total amount of thermal radiation, therefore, drastically increases with higher temperatures.
At temperatures below about 900°F (482°C), most blackbody radiation is in the infrared part of the electromagnetic spectrum. The wavelengths of this radiation are outside the range for human visual perception, which explains why people can’t see room-temperature objects during the night. Very hot objects begin to emit substantially in the visible range of light, however. This explains why people can see burning wood or a distant star during the night. The light from these hot objects is not primarily being reflected from other sources, so it is blackbody radiation.
As mentioned, real-world objects always reflect some of the incident light they receive. Individuals interpret this reflected light as being able to “see” an object; people can see cool objects because they reflect the blackbody radiation of hotter objects. Whether the visible light waves come from the sun or a light bulb, they reach our eyes only after bouncing off a cool object. Without this original blackbody radiation, cool objects would appear black—which is just what individuals notice at nighttime without any hot objects in sight. If humans were able to see in the infrared, they would be able to see even room-temperature objects.