All electromagnetic radiation has similar wavelike properties differing only in wavelength. Electromagnetic waves range in wavelength from very long (e.g., electric power line radiation at 60 Hz) to very short (e.g., gamma ray radiation). This entire range is called the electromagnetic spectrum. The spectrum shown in Figure 1-11 is divided by the practical applications for given ranges of frequencies that are set through convention by the sources and detection devices.

Of primary interest to photonics is the region from infrared to ultraviolet. However, each regime has some utility. Rotating generators and power lines generate low-frequency waves. These wavelengths are on the order of 10⁵ to 10⁸ meters. Heinrich Hertz produced radio waves in a very useful region of wavelengths ranging from 0.3 to 10⁵ meters. Television and radio broadcasting bands are found in lower wavelengths. The microwave regime ranges from 0.01 to 0.3 meter and provides the radar and satellite communication bands. The infrared region, from 1 mm to 30 mm, was first detected by Sir William Herschel in 1800. This region is subdivided into five regions: very near (1–3 mm), near (3–5 mm), mid (5–6 mm), far (6–15 mm), and very long (15–30 mm) infrared. Just as the ear cannot hear above or below certain frequencies, the human eye cannot detect light outside a small range of wavelengths (0.76–0.49 mm). The ultraviolet region is a higher-energy region discovered by Johann Ritter. It triggers many chemical reactions and is what ionizes the upper atmosphere, creating the ionosphere. Wilhelm Röntgen discovered the X-ray regime in 1895. Its wavelength ranges from 10^–8 to 10^–11 meters. With its high energy, it can penetrate flesh and provide an image of higher-density material such as bones. Gamma rays represent the smallest wavelength (less than 10^–13 meter). They exhibit particle-like properties with great energy and are emitted by the sun, linear and particle beam accelerators, and nuclear processes.

Figure 1-11  Electromagnetic spectrum

White light is a mixture of light of different colors. Each of these colors has a different wavelength and, when passed through a transparent medium, refracts differently. Thus, a prism can separate white light into its component colors, as shown in Figure 1-12.

Figure 1-12  Separation of light into component colors

The colors displayed in visible light are categorized by wavelength. Table 1-2 gives the wavelengths of these colors. An arrangement showing the different components of light, with the wavelengths of the components in order, is called the spectrum of the light.