What we typically think of as “light” is really electromagnetic radiation that our eyes can see. We perceive the world in the colors of the rainbow, red through violet. But these colors of light are actually a very small portion of the electromagnetic.
Our eyes are capable of seeing only a very narrow region of the eletromagnetic spectrum, and we need special instruments to extend our vision beyond the limitations of the unaided eye. As the energy of light changes, so too does its interaction with matter. Materials that are opaque at one wavelength may be transparent at another. A familiar example of this phenomenon is the penetration of soft tissue by X-rays.
What is opaque to visible light becomes transparent to reveal the bones within.
Extending human vision with electronic imaging is one of the most powerful techniques available to science and industry, articularly when it enables us to see light in the infrared, or IR portion of the spectrum. Infrared means “below red,” as infrared light has less energy then red light. We typically describe light energy in terms of wavelength, and as the energy of light decreases, its
wavelength gets longer. Infrared light, having less energy than visible light, has a correspondingly longer wavelength. The infrared portion of the spectrum ranges in wavelength from 1 to 15um, or about 2 to 30 times longer wavelength (and 2 – 30 times less energy) than visible light.
Infrared light is invisible to the unaided eye, but can be felt as heat on one’s skin. Warm objects emit infrared light, and the hotter the object, the shorter the wavelength of IR light emitted. This IR “glow” enables rescue workers equipped with longwave IR sensors to locate a lost person in a deep forest in total darkness, for example. Infrared light can penetrate smoke and fog better than visible light, revealing objects that are normally obscured. It can also be used to detect the presence of excess heat or cold in a piece of machinery or a chemical reaction.