Radiation

The third and last form of heat transfer we shall consider is that of radiation, which in this context means light (visible or not). This is the means by which heat is transferred, for example, from the sun to the earth through mostly empty space - such a transfer cannot occur via convection nor conduction, which require the movement of material from one place to another or the collisions of molecules within the material.

Often the energy of heat can go into making light, such as that coming from a hot campfire. This light, being a wave, carries energy, as we saw in the last chapter, and so can move from one place to another without requiring an intervening medium. When this light reaches you, part of the energy of the wave gets converted back into heat, which is why you feel warm sitting beside a campfire. Some of the light can be in the form of visible light that we can see, but a great deal of the light emitted is infrared light, whose longer wavelength is detectable only with special infrared detectors. The hotter the object is, the less infrared light is emitted, and the more visible light. For example, human beings, at a temperature of about 37 ^o Celsius, emit almost exclusively infrared light, which is why we don't see each other glowing in the dark. On other hand, the hot filament of a light bulb emits considerably more visible light. We shall discuss in more detail the nature of light in Chapter 10.