So what is infrared light and what makes the world of infrared photography so special...
What we see - the world of visible light - is just a small part of what is known as the electromagnetic spectrum of radiation, which ranges all the way from gamma rays at very short wavelengths (and correspondingly dangerously high energies) to low energy radio waves with wavelengths of over a mile.
The wavelengths that our eye can see are limited to those between around 400-700 nm (a nanometer is one millionth of a millimeter), a tiny fraction of this huge spectrum. The infrared range of the electromagnetic spectrum lies beyond the red. This portion of the spectrum is invisible to human eyes and was first discovered by Sir Frederick William Herschel (1738-1822) in 1800.
Infrared includes all radiations between wavelengths just beyond those of the deepest reds of the visible spectrum (700 nm) and microwaves (100,000+ nm), which are used for cooking in microwave ovens. Although infrared covers a vast part of the spectrum, the portion of it that can be used to expose photographic emulsions is actually quite small. The area extends from about 700 nm to about 1200 nm, but most amateur and commercial infrared films are only sensitized to about 900 nm.
The peculiarities of infrared photography lie in :
Image courtesy of Lifepixel.com
These properties make infrared photography very special and permit it to be used as an important adjunct to photography by normal light.