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.

### THE COMPONENTS OF VISIBLE LIGHT

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.

### THE FULL RANGE OF RADIATION

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.