Obviously, it is not possible to touch every item that this vast subject holds, and I depend on readers to open up more frontiers. We barely scratched the surface about microwaves, but have to move to more energetic bands.
The infrared band lies just above the microwave, and just below the visible. Issac Newton discovered it by taking a thermometer and placing it in different spots on a projection of a spectrum of sunlight provided by a simple glass prism. He noted that there were area of higher temperature below the violet (now called ultraviolet) and above the red (now called infrared). These terms come from the natural distribution of a solar spectrum by a prism, the red being at the top of the graph.
So, here once again we go.
Infrared radiation is all electromagnetic radiation between microwaves and the longest wavelength of red light that may be sensed by the human eye, that sense beginning around 400 nm (nanometers). Infrared can be sensed by the skin indirectly, as it causes heating and thus stimulates the nerve endings. Some Crotalid serpents have discreet infrared sensors ("pits", hence the name "pit vipers") that can triangulate on infrared and use it nearly as we do visible wavelengths.
I suppose now is as good a time as any to talk about "blackbody radiation", also known as "cavity radiation". This radiation is electromagnetic, and the wavelength depends solely on the temperature of the body that radiates it. This observation actually was one of the reasons that Max Planck came up with the earliest idea of quantum theory, because the original idea led to the "ultraviolet catastrophe", wherein the mathematical model collapsed. For tonight, it is sufficient to know that the average maximum of radiative output is a function of temperature. The brilliant statistical physicist Boltzmann also contributed to this development.
In a nutshell, blackbody theory says that both the intensity and the frequency of EMR radiated from a body has to do in a nonlinear way with temperature, and the relationship is complex. That is why we do not see much radio nor microwave from a warm body, just not enough of either factor. In any event, this is not a dissertation about blackbody radiation, but the concept is necessary for further developments.
Infrared is, optically, much like visible light. It pretty much follows the same rules insofar as reflection and refraction, we just can not see it with our feeble eyes. But it has one unique property.
This changes with infrared. At temperatures sufficient to produce it, there is enough both in intensity and wavelength to measure. I will go back to the campfire analogy: coals produce little visible light but will burn you at a distance if you hold your hand close enough for long enough.
Blackbody radiation is essentially a continuum, with the wavelength and intensity peaking depending on temperature, with no special bands of high intensity here or there.
But in this spectrum there are both continua and discreet emissions. The continua come from hot bodies, and the discreet from atomic vibrations within molecules. In fact, infrared spectra are a fingerprint for molecules unlike any other.
There are selection rules for infrared activity that have to do with quantum mechanics. The main one is that the dipole moment of the molecule has to change to be active. For example, carbon dioxide has the following vibrations: a symmetrical stretch for each oxygen to flee or approach the central carbon at the same rate (infrared inactive), an asymmetrical one where one oxygen approaches as the other one departs (infrared active), the "scissoring" one where the oxygens open and close (the symmetric one is inactive, but the one that looks like a bird flapping is active). This is just a very simple example.
Broadband IR is used for military purposes to look for heat, meaning troops or engines, and also used by civilian law enforcement to track down criminals, or at least that is what they say. It can be used to track down anyone.
Well, this is long enough. I will stick around for questions, flames, and comments for a bit. I will also crosspost at Politicook.net. Warmest regards, Doc.