CoachEach monochromatic radiation is characterized by one number, its wavelength , measured in nanometers (). The wavelength is the identity card of the radiation: give and you have said everything there is to say about the pure color. The eye responds to wavelengths between about (violet) and (red) — the visible spectrum. Beyond the two ends the radiation continues, invisible to us: ultraviolet below , infrared above . What the wavelength actually measures — the length of one ripple of a light wave — is explained in Chapter 8; in this chapter, the number itself is what matters.
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Example 2.4 (Reading wavelengths)
A green laser pointer emits at : inside the visible range, in the green. A germicidal mercury lamp emits at : ultraviolet, invisible (and dangerous to the eye precisely because invisible). The diode of a television remote emits at : infrared — point it at a phone camera, whose sensor sees a little beyond , and the invisible flashes appear on the screen.
Example 2.17 (A flask of sodium vapor)
White light crosses a flask containing cool sodium vapor. The transmitted spectrum is a full rainbow with one dark line at — precisely where a sodium lamp shines brightest. Looking at the flask from the side in a dark room, one sees the stolen light re-emitted: a faint yellow-orange gleam at that same wavelength.