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A '''black body''' is an hypothetical object that absorbs all [[light]] [[radiation]] incident upon it. Because light cannot pass through or reflect from a black body, all radiation incident from such an object is emitted from the object itself. Therefore, the idea of a black body is a useful theoretical tool for studying thermal emission apart from other radiative effects. The idea also has great practical use, since though in practice no object is a perfect absorber of light, many real objects (such as the Sun, or a hot filament in a lightbulb) emit so much more radiation than they reflect or transmit that they can be described extremely successfully with a black body model.  

A '''black body''' is an hypothetical object that absorbs all [[light]] [[radiation]] incident upon it. Because light cannot pass through or reflect from a black body, all radiation incident from such an object is emitted from the object itself. Therefore, the idea of a black body is a useful theoretical tool for studying thermal emission apart from other radiative effects. The idea also has great practical use, since though in practice no object is a perfect absorber of light, many real objects (such as the Sun, or a hot filament in a lightbulb) emit so much more radiation than they reflect or transmit that they can be described extremely successfully with a black body model.  

Like all light, the light emitted by a black body has a particular energy which depends upon its [[frequency]], or how quickly the light waves oscillate. In the visible spectrum, we see this energy difference as colour - our visual systems are tuned to interpret light of a certain frequency as red, of a different frequency as blue, etc. There is also an infinite range of frequencies that are too high or too low for us to see, although they still transmit energy. For instance, [[radio waves]] are very low frequency light, while [[X-rays]] have very high frequencies. Black bodies emit a characteristic range of these frequencies, whose exact shape depends on the black body's [[temperature]. This is why objects glow different colours as they are heated - the peak frequency of emission rises with temperature. In fact, all objects, even apparently dark ones, emit light of some frequency. However, most of the objects around us are too cold for their peak frequency to be in the visible range and are instead in the [[infrared]] part of the spectrum. This is one of the reasons why infrared goggles make it possible to see in the dark: the goggles pick up the low-energy "glow" of room-temperature objects.

Like all light, the light emitted by a black body has a particular energy which depends upon its [[frequency]], or how quickly the light waves oscillate. In the visible spectrum, we see this energy difference as colour - our visual systems are tuned to interpret light of a certain frequency as red, of a different frequency as blue, etc. There is also an infinite range of frequencies that are too high or too low for us to see, although they still transmit energy. For instance, [[radio waves]] are very low frequency light, while [[X-rays]] have very high frequencies. Black bodies emit a characteristic range of these frequencies, whose exact shape depends on the black body's [[temperature]]. This is why objects glow different colours as they are heated - the peak frequency of emission rises with temperature. In fact, all objects, even apparently dark ones, emit light of some frequency. However, most of the objects around us are too cold for their peak frequency to be in the visible range and are instead in the [[infrared]] part of the spectrum. This is one of the reasons why infrared goggles make it possible to see in the dark: the goggles pick up the low-energy "glow" of room-temperature objects.

The characteristic range of frequencies, or "frequency spectrum", emitted by a black body can be experimentally measured by heating a black body up to a known temperature, then measuring the intensity ("brightness") of the energy that the black body emits at each frequency. The means to do this were available to late 19th century physicists, and thus the shape of this curve was known then. However, these physicists could not calculate the shape of this curve from theory. This is because an important result in classical mechanics called the "equipartition theorem" states that, on average, the energy of a large group of objects must be evenly distributed between all the different ways that those objects can store energy. For example, a system of thousands of pendulums hanging from rubber bands would, on average, spend half of its kinetic energy on the swinging of the pendulums, and the other half on the bouncing of the rubber bands. If the pendulums were also able to rotate, then the energy would be divided evenly three ways between swinging, bouncing, and rotation.  

The characteristic range of frequencies, or "frequency spectrum", emitted by a black body can be experimentally measured by heating a black body up to a known temperature, then measuring the intensity ("brightness") of the energy that the black body emits at each frequency. The means to do this were available to late 19th century physicists, and thus the shape of this curve was known then. However, these physicists could not calculate the shape of this curve from theory. This is because an important result in classical mechanics called the "equipartition theorem" states that, on average, the energy of a large group of objects must be evenly distributed between all the different ways that those objects can store energy. For example, a system of thousands of pendulums hanging from rubber bands would, on average, spend half of its kinetic energy on the swinging of the pendulums, and the other half on the bouncing of the rubber bands. If the pendulums were also able to rotate, then the energy would be divided evenly three ways between swinging, bouncing, and rotation.