Black-body radiation is emitted by objects based on their temperature. A black body absorbs all incoming radiation and emits radiation only based on its temperature. Real-world objects reflect some radiation, but hot black bodies emit visible light while cool ones emit infrared. Thermal radiation is proportional to an object’s absolute temperature to the fourth power. Most black body radiation is in the infrared spectrum, making cold objects invisible to humans. Humans see cold objects because they reflect black-body radiation from hotter objects.
Black-body radiation is thermal radiation emitted by, rather than reflected from, an object. Physically, black-body radiation is always mixed with reflected radiation, because all real-world objects reflect some of the electromagnetic waves they receive. However, a hypothetical black body would emit radiation dependent only on its temperature. Hot black bodies would emit basically in the visible range of light and, therefore, be visible to humans. Cool blackbodies would mostly emit 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 from the body 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 a portion of incoming radiation, although this amount can vary. Black bodies are so named because, not having reflected incident light waves, they would appear black when cold.
All objects, including black bodies, emit an amount of thermal radiation that depends on the object’s temperature. In fact, this quantity is proportional to the absolute temperature of an object to the fourth power. Doubling the absolute temperature of an object, for example, by bringing an oven from room temperature to about 600° Fahrenheit (315° Celsius), will cause 16 times more heat radiation. The total amount of thermal radiation, therefore, increases dramatically as temperatures rise.
At temperatures below about 900°F (482°C), most black body 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 cannot see objects at room temperature at night. However, very hot objects start to emit basically in the visible range of light. This explains why people can see burning wood or a distant star at night. The light from these hot objects is not mainly reflected from other sources, so it is black body radiation.
As mentioned, real-world objects always reflect some of the incident light they receive. Individuals interpret this reflected light as the ability to “see” an object; people can see cold objects because they reflect black-body radiation from hotter objects. Whether visible light waves come from the sun or from a light bulb, they only reach our eyes after bouncing off a cold object. Without this original black-body radiation, cold objects would appear black, which is just what individuals notice at night with no hot objects in sight. If humans were able to see in the infrared, they would be able to see objects at room temperature as well.
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