2.3 The Electromagnetic Spectrum
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 2.7, we can separate a beam of white light into a rainbow of these basic colors—called a spectrum (plural: spectra)—by passing it through a prism. This experiment was first reported by Isaac Newton more than 300 years ago. In principle, the original beam of white light could be recovered by passing the spectrum through a second prism to recombine the colored beams.
The Components of Visible Light
What determines the color of a beam of light? The answer is its frequency (or, equivalently, its wavelength)—we see different colors because our eyes react differently to electromagnetic waves of different frequencies. Red light has a frequency of roughly corresponding to a wavelength of about Violet light, at the other end of the visible range, has nearly double the frequency——and (since the speed of light is the same in either case) just over half the wavelength— The other colors we see have frequencies and wavelengths intermediate between these two extremes.
Scientists often use a unit called the nanometer (nm) when describing the wavelength of light (see Appendix 2). There are nanometers in one meter. An older unit called the angstrom is also widely used by many astronomers and atomic physicists, although the nanometer is now 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.
Figure 2.8 plots the entire range of electromagnetic radiation. To the low-frequency, long-wavelength side of visible light lies radio and infrared radiation. Radio frequencies include radar, microwave radiation, and the familiar AM, FM, and TV bands. We perceive infrared radiation as heat. To the high-frequency, short-wavelength side of visible light lies 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 our resorting to surgery. Gamma rays are the shortest-wavelength radiation. They are often associated with radioactivity and are invariably damaging to any living cells they encounter.
All these spectral regions, including the visible, 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 types of electromagnetic radiation are basically the same and all move at the same speed—the speed of light c.
Figure 2.8 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. Scientists often disagree on the “correct” way to display wavelengths and frequencies in diagrams of this type. This book consistently adheres to the convention that frequency increases toward the right. Notice that the wavelength and frequency scales in Figure 2.8 do not increase by equal increments of 10. Instead, successive values marked on the horizontal axis differ by factors of 10—each successive value 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. Throughout the text we will often find it convenient to use such a scale in order to compress a wide range of some quantity onto a single easy-to-view plot.
Figure 2.8 shows wavelengths extending from the height of mountains for radio radiation to the diameter 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 objects 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 2.8—to every astronomical image presented in this text.
Only a small fraction of the radiation arriving at our planet actually reaches Earth’s surface 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. Earth’s atmospheric opacity is plotted along the wavelength and frequency scales at the bottom of Figure 2.8. Where the shading is greatest, no radiation can get in or out—the energy is completely absorbed by atmospheric gases. Where there is no shading at all, our atmosphere is almost totally transparent. Note that there are just 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. Over the rest of the spectrum, however, the atmosphere is opaque. As a result, ultraviolet, X-ray, and gamma-ray observations can be made only from above the atmosphere, from orbiting satellites.