All materials that contain heat emit electromagnetic radiation due to oscillations within their atomic structure. An object at 0K (-273°C) would emit no such radiation as it contains no heat energy - in theory all its molecules would be stationary. If it were heated up slightly, the atoms it contains would begin to vibrate, thus inducing an electromagnetic field around them which interacts with the fields of other nearby atoms. This results in the emission of small amounts of low energy long-wave electromagnetic radiation as free and valence electrons within the atomic structure exchange energy.
With all this vibration, electrons can be dislodged from atoms and become free electrons, jostled about violently within these oscillating fields. As these electrons move about, they collide with atoms, loosing a small amount of kinetic energy. This energy is not actually lost, it temporarily raises the orbit of circling electrons within the impacted atom, raising its overall energy level. This puts the atom in a temporarily unstable state. The displaced electron quickly returns to its original orbit, releasing the extra energy as a photon of electromagnetic radiation.
The hotter the object becomes, the faster some of its atoms vibrate and the more energetic its released photons. As the speed or frequency of molecular vibration is linearly related to wavelength, the temperature of an object therefore determines the wavelength (and hence the frequency) of its radiated energy. This is known as colour temperature and simply means that the higher the temperature of an object, the further up the spectrum its electromagnetic radiation occurs.
Colour Temperature
From practical experience we know that very hot objects glow a dull red (around 873K) and that very-very hot objects glow white (around 3000K), hence the colloquial terms red-hot and white-hot. If you look at the spectrum of visible light, however, theoretically the very-very hot object should really glow violet or at least blue.
In fact, the object does emit light at blue and violet frequencies, but it is also emitting light energy at a range of lower frequencies as well. If the amount of blue light is roughly the same as the amounts of green, yellow and red light, then that is the definition of white light (i.e.: contains all frequencies). This continuous spectrum of radiation results from the irregular excitation of very large numbers of free electrons at different states within the solid, only some of which will be at the highest temperature.
The CIE Chromaticity Chart shows the relationship of colour temperature and chromaticity. The visible emissions of hot objects follow the black line towards the centre of the diagram. Thus it is impossible to heat an object and get it to glow green, the best you will get is yellow at around 2500K. Note also that, when a gas flame glows blue, this is a special case called pyroluminescence in which the rapid oxidation of the gas emits only a small range of light frequencies at the upper end of the spectrum. This is due to the nature of the gas itself and its homogeneity. If you used the heat from that same gas flame on a piece of metal, you would find that the metal would begin glowing red and eventually glow white-hot, but never blue.
Spectral Content
That hot objects emit radiation at all frequencies beneath their maximum energy is true, however the emissions are not usually equal. What you actually get is something closer to a standard distribution (or a bell curve) where some atoms are a little hotter and some a little cooler than the average temperature. In a white-hot piece of metal it would be unlikely that you would find atoms still at quite low temperatures, so there would be very little low-energy/low-frequency emissions.
Figure 3 below shows the relative electromagnetic emissions from a range of sources at various temperatures. Note that the Earth, at an average temperature of around 20°C, emits only long wave infra-red radiation. Only at a temperature approaching 1500K does molten glass begin to give off some amount of visible light. Also, whilst a normal fire does give off light, this is actually due to pyroluminescent emissions from the burning gas, not the temperature at which it burns. The glowing embers in the remains of a fire more accurately reflect its colour temperature.
It is interesting to note that nearly two-thirds (66%) of the energy radiated by an incandescent lamp is in the infrared band (i.e.: heat), with only one-third (33%) being visible light. You can understand this when you place your hand near an exposed bulb and feel the emitted heat. Contrast this with sunlight, where around half is visible light and less than 40% is infrared (the other 10% is made up of ultraviolet frequencies above the visible range). This makes sunlight and daylight very efficient forms of lighting as you actually get more light for less heat input than most artificial lighting systems.
For more information on the effects of radiation at different frequencies, see the electromagnetic spectrum and greenhouse effect topics.
Useful References
- Colour Temperature - Wikipedia
- http://en.wikipedia.org/wiki/Color_temperature
