ABSTRACT
In order to use spectroscopy to study matter through its interaction with electromagnetic radiation, we first have to understand radiation. The electromagnetic spectrum spans a frequency range that varies by many orders of magnitude, as shown in Figure 2.1. We broadly refer to this radiation as light, even though only a very small portion of the spectrum is visible to the human eye. The frequency ν and the wavelength λ of light are inversely proportional to one another: c = λν is the speed of light in a vacuum, 2.9979 × 108 m/s. The spectrum ranges from the low-energy radiofrequency region, where the frequencies are in the MHz to kHz range (Hz = Hertz, a unit of frequency equivalent to cycles per second or s−1), to the high frequency gamma-ray region, characterized by frequencies as high as 1022 Hz. The corresponding wavelengths go from thousands of kilometers down to less than 10−15 m. The wavelengths of visible light, which span only a tiny fraction of an essentially limitless spectrum, range from about 400 to 700 nm, on going from violet to red. These wavelengths correspond to frequencies from 7.5 × 1014 Hz (violet) to 4.3 × 1014 Hz (red), or in wavenumber units, from about 25,000 cm−1 to 14,000 cm−1.
