White light is a mixture of colors, which we conventionally divide into six major hues—red, orange, yellow, green, blue, and violet. As shown in Figure 3.10, we can identify each of these basic colors by passing light through a prism. In principle, the original beam of white light could be restored by passing the entire red-to-violet range of colors—called a spectrum (plural: spectra)—through a second, oppositely oriented prism to recombine the colored beams. This experiment was first reported by Isaac Newton over 300 years ago.
Figure 3.10 While passing through a prism, white light splits into its component colors, spanning red to violet in the visible part of the electromagnetic spectrum. The slit narrows the beam of radiation. The image on the screen is just a series of different-colored images of the slit.
What determines the color of a beam of light? The answer is its wavelength (or, equivalently, its frequency). We see different colors because our eyes react differently to electromagnetic waves of different wavelengths. A prism splits a beam of light up into the familiar "rainbow" of colors because light rays of different wavelengths are bent, or refracted, slightly differently as they pass through the prism—red light the least, violet light the most. Red light has a frequency of roughly 4.3 
1014 Hz, corresponding to a wavelength of about 7.0 10 -7 m. Violet light, at the other end of the visible range, has nearly double the frequency—7.5 1014 Hz—and (since the speed of light is the same in either case) just over half the wavelength—4.0 10 -7 m. The other colors we see have frequencies and wavelengths intermediate between these two extremes, spanning the entire visible spectrum shown in Figure 3.10; radiation outside this range is invisible to human eyes.
Astronomers often use a unit called the nanometer (nm) when describing the wavelength of light (see Appendix 2). There are 109 nanometers in 1 meter. An older unit called the angstrom (1Å - 10 -10 m - 0.1 nm) is also widely used. (The unit is named after the nineteenth-century Swedish physicist Anders Ångstrom—pronounced "ongstrem.") However, in SI units, the nanometer is preferred. Thus, the visible spectrum covers the wavelength range from 400 to 700 nm (4000 to 7000 Å). The radiation to which our eyes are most sensitive has a wavelength near the middle of this range, at about 550 nm (5500 Å), in the yellow-green region of the spectrum. It is no coincidence that this wavelength falls within the range of wavelengths at which the Sun emits most of its electromagnetic energy—our eyes have evolved to take greatest advantage of the available light.
Figure 3.11 plots the entire range of electromagnetic radiation, illustrating the relationships among the different "types" of electromagnetic radiation listed earlier. We see that the only characteristic that distinguishes one from another is its wavelength, or frequency. To the low-frequency, long-wavelength side of visible light lie radio and infrared radiation. Radio frequencies include radar, microwave radiation, and the familiar AM, FM, and TV bands. We perceive infrared radiation as heat. At higher frequencies (shorter wavelengths) are the domains of ultraviolet, X-ray, and gamma-ray radiation. Ultraviolet radiation, lying just beyond the violet end of the visible spectrum, is responsible for suntans and sunburns. X-rays are perhaps best known for their ability to penetrate human tissue and reveal the state of our insides without resorting to surgery. Gamma rays are the shortest-wavelength radiation. They are often associated with radioactivity and are invariably damaging to living cells they encounter.
Figure 3.11 The entire electromagnetic spectrum.
All these spectral regions, including the visible spectrum, collectively make up the electromagnetic spectrum. Remember that, despite their greatly differing wavelengths and the very different roles they play in everyday life on Earth, all are basically the same phenomenon, and all move at the same speed—the speed of light, c.
Figure 3.11 is worth studying carefully, as it contains a great deal of information. Note that wave frequency (in hertz) increases from left to right, and wavelength (in meters) increases from right to left. These wave properties behave in opposite ways because, as noted earlier, they are inversely related. When picturing wavelengths and frequencies, this book will adhere to the convention that frequency increases toward the right. Notice that the wavelength and frequency scales in Figure 3.11 do not increase by equal increments of 10. Instead, successive values marked on the horizontal axis differ by factors of 10—each is 10 times greater than its neighbor. This type of scale, called a logarithmic scale, is often used in science in order to condense a very large range of some quantity into a manageable size. Had we used a linear scale for the wavelength range shown in Figure 3.11, the figure would have been many light years long! Throughout the text we will often find it convenient to use a logarithmic scale in order to compress a wide range of some quantity onto a single, easy-to-view plot.
Figure 3.11 shows that wavelengths extend from the size of mountains for radio radiation to the size of an atomic nucleus for gamma-ray radiation. The box at the upper right emphasizes how small the visible portion of the electromagnetic spectrum is. Most objects in the universe emit large amounts of invisible radiation. Indeed, many of them emit only a tiny fraction of their total energy in the visible range. A wealth of extra knowledge can be gained by studying the invisible regions of the electromagnetic spectrum. To remind you of this important fact and to identify the region of the electromagnetic spectrum in which a particular observation was made, we have attached a spectrum icon—an idealized version of the wavelength scale in Figure 3.11—to every astronomical image presented in this text.
Our eyes are sensitive to only a minute portion of the many different kinds of radiation known. In addition, only a small fraction of the radiation produced by astronomical objects actually reaches our eyes, in part because of the opacity of Earth's atmosphere. Opacity is the extent to which radiation is blocked by the material through which it is passing—in this case, air. The more opaque an object is, the less radiation gets through it. Opacity is the opposite of transparency. At the bottom of Figure 3.11, the atmospheric opacity is plotted along the wavelength and frequency scales. The extent of shading is proportional to the opacity. Where the shading is greatest, no radiation can get in or out. Where there is no shading at all, the atmosphere is almost completely transparent, so that extraterrestrial radiation can reach Earth's surface, and terrestrial radiation from human transmissions can pass virtually unhindered into space.
What causes opacity to vary along the spectrum? Certain atmospheric gases are known to absorb radiation very efficiently at some wavelengths. For example, water vapor (H2O) and oxygen (O2) absorb radio waves having wavelengths less than about a centimeter, whereas water vapor and carbon dioxide (CO2) are strong absorbers of infrared radiation. Ultraviolet, X-ray, and gamma-ray radiation are completely blocked by the ozone layer high in Earth's atmosphere (see Sec. 7.3). A passing but unpredictable source of atmospheric opacity in the visible part of the spectrum is the blockage of light by atmospheric clouds. In addition, the interaction between the Sun's ultraviolet radiation and the upper atmosphere produces a thin, electrically conducting layer at an altitude of about 100 km. The ionosphere, as this layer is known, reflects long-wavelength radio waves (wavelengths greater than about 10 m) as well as a mirror reflects visible light. In this way, extraterrestrial waves are kept out, and terrestrial waves—such as those produced by AM radio stations—are kept in. (That is why it is possible to transmit some radio frequencies beyond the horizon—the broadcast waves bounce off the ionosphere.)
The effect of all this blockage is that there are only a few windows, at well-defined locations in the electromagnetic spectrum, where Earth's atmosphere is transparent. In much of the radio and in the visible portions of the spectrum, the opacity is low, and we can study the universe at those wavelengths from ground level. In parts of the infrared range, the atmosphere is partially transparent, so we can make certain infrared observations from the ground. Moving to the tops of mountains, above as much of the atmosphere as possible, improves observations. In the rest of the spectrum, however, the atmosphere is opaque. Ultraviolet, X-ray, and gamma-ray observations can be made only from above the atmosphere, from orbiting satellites.
