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Chapter 1:
Infrared and Ultraviolet: The Edges of the Rainbow

The near-infrared (near-IR) and near-ultraviolet (near-UV) wavebands flank the visible spectrum of light. Their relationship to the visible waveband is shown in Fig. 1.1, along with the short wave infrared (SWIR) waveband, described shortly. The chemistry of our eyes bounds our color vision; near-IR photons do not have enough energy to stimulate our eyes, and the lenses of our eyes block near-UV light. Though we cannot see these "colors" of light, they are close cousins to the ones we can see. The optical properties of glass and photographic film in the near-IR and near-UV wavebands are similar enough to their properties in the visible waveband that scientists have been able to image in the near-IR and near-UV wavebands using special filters and films for over a century. These modified camera systems reveal a surprising view of familiar objects and materials. The changed appearance of the familiar or the revelation of things unseen is the essence of "alien vision" (i.e., the imaging of the world in wavebands of light that human eyes cannot see). Alien vision suggests extraterrestrial beings that see with invisible light, yet there are familiar creatures around that see light that we cannot; for instance, butterflies, birds, and honeybees are sensitive to near-UV light. Ultraviolet vision is quite common in the animal kingdom, particularly among invertebrates. In fact, there are many examples of markings and patterns on animals and plants that appear to act as signals or cues to animals with near-UV vision. These markings were unknown until the advent of ultraviolet imaging technology in the early part of the 20th century.

Light in both the near-IR and the near-UV wavebands is easy to generate with a prism: let a narrow shaft of sunlight enter a darkened room, place a glass prism on a table so as to intersect the shaft of light, and place a white screen behind the prism. The prism bends the light rays according to their color, with red bent the least and violet bent the most. This wavelength-dependent refraction is a property of the glass known as dispersion. The white beam of light becomes a rainbow projected onto the screen, with the familiar colors arranged in order of increasing frequency and decreasing wavelength: red, orange, yellow, green, blue and violet, as shown in Fig. 1.2. As we move along the pattern, we pass through orange, then red, and then the red becomes a deeper shade of red, like the color of dying embers in a fire. Finally, the red light appears to fade out completely; we have reached the near-IR region of the spectrum. Our eyes are unable to perceive this light, although we can feel its heat if the source is bright enough.

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