2.1 Information from the Skies

Figure 2.1 shows our nearest large galactic neighbor, which lies in the constellation Andromeda. On a dark, clear night, far from cities or other sources of light, the Andromeda Galaxy, as it is generally called, can be seen with the naked eye as a faint, fuzzy patch on the sky, comparable in diameter to the full Moon. Yet the fact that it is visible from Earth belies this galaxy's enormous distance from us. It lies roughly 2.5 million light-years away. An object at such a distance is truly inaccessible in any realistic human sense. Even if a space probe could miraculously travel at the speed of light, it would need two and a half million years to reach this galaxy and two and a half million more to return with its findings. Considering that civilization has existed on Earth for fewer than 10,000 years (and its prospects for the next 10,000 are far from certain), even this unattainable technological feat would not provide us with a practical means of exploring other galaxies—or even the farthest reaches of our own galaxy, several tens of thousands of light-years away.

	Figure 2.1 Andromeda Galaxy The pancake-shaped Andromeda Galaxy is about 2.5 million light-years away and contains a few hundred billion stars. (T. Hallas)

Light and RadiationHow do astronomers know anything about objects far from Earth? How can we obtain detailed information about any planet, star, or galaxy too distant for a personal visit or any kind of controlled experiment? The answer is that we use the laws of physics, as we know them here on Earth, to interpret the electromagnetic radiation emitted by these objects.Radiation is any way in which energy is transmitted through space from one point to another without the need for any physical connection between those two locations. The term electromagnetic means that the energy is carried in the form of rapidly fluctuating electric and magnetic fields (see Section 2.2). Virtually all we know about the universe beyond Earth's atmosphere has been gleaned from analysis of electromagnetic radiation received from afar.
Visible light is the particular type of electromagnetic radiation to which the human eye happens to be sensitive. But there is also invisible electromagnetic radiation, which goes completely undetected by our eyes. Radio, infrared, and ultraviolet waves, as well as X rays and gamma rays, all fall into this category. You should recognize that, despite the different names, the words light, rays, electromagnetic radiation, and waves really all refer to the same thing. The names are just historical accidents, reflecting the fact that it took many years for scientists to realize that these apparently very different types of radiation are in reality one and the same physical phenomenon. Throughout this text, we will use the general terms “light” and “electromagnetic radiation” more or less interchangeably.
Wave MotionAll types of electromagnetic radiation travel through space in the form of waves. To understand the behavior of light, then, we must know a little about this kind of motion. Simply stated, a wave is a way in which energy is transferred from place to place without physical movement of material from one location to another. In wave motion, the energy is carried by a disturbance of some sort that occurs in a distinctive, repeating pattern. Ripples on the surface of a pond, sound waves in air, and electromagnetic waves in space, despite their many obvious differences, all share this basic defining property.

Figure 2.2 Water Wave The passage of a wave across a pond causes the surface of the water to bob up and down, but there is no movement of water from one part of the pond to another. Here waves ripple out from the point where a pebble hit the water to the point where a twig is floating. The inset shows a series of “snapshots” of the pond surface as the wave passes by. The points numbered 1 through 5 represent surface locations that bob up and down with the passage of the wave.

As a familiar example, imagine a twig floating in a pond (Figure 2.2). A pebble thrown into the pond at some distance from the twig disturbs the surface of the water, setting it into up-and-down motion. This disturbance propagates outward from the point of impact in the form of waves. When the waves reach the twig, some of the pebble's energy is imparted to it, causing the twig to bob up and down. In this way, both energy and information—the fact that the pebble entered the water—are transferred from the place where the pebble landed to the location of the twig. We could tell just by observing the twig that a pebble (or some small object) had entered the water. With a little additional physics, we could even estimate the pebble's energy.A wave is not a physical object. No water traveled from the point of impact of the pebble to the twig—at any location on the surface, the water surface simply moved up and down as the wave passed. What, then, does move across the pond surface? The answer is that the wave is thepattern of up-and-down motion, and it is this pattern that is transmitted from one point to the next as the disturbance moves across the water.
Figure 2.3 shows how wave properties are quantified. The wave period is the number of seconds needed for the wave to repeat itself at some point in space. The wavelength is the number of meters needed for the wave to repeat itself at a given moment in time. It can be measured as the distance between two adjacent wave crests, two adjacent wave troughs, or any other two similar points on adjacent wave cycles (for example, the points marked “X” in Figure 2.3). The maximum departure of the wave from the undisturbed state—still air, say, or a flat pond surface—is called itsamplitude.

	Figure 2.3 Wave Properties Representation of a typical wave, showing its direction of motion, wavelength, and amplitude. In one wave period the entire pattern shown moves one wavelength to the right.

The number of wave crests passing any given point per unit time is called the wave'sfrequency. If a wave of a given wavelength moves at high speed, then many crests pass by per second and the frequency is high. Conversely, if the same wave moves slowly, then its frequency is low. The frequency of a wave is just one divided by the wave's period:

Frequency is expressed in units of inverse time (cycles per second), called hertz (Hz) in honor of the nineteenth-century German scientist Heinrich Hertz, who studied the properties of radio waves. A wave with a period of five seconds has a frequency of (1/5) 

Basic Wave Properties

A wave moves a distance equal to one wavelength in one wave period. The product of wavelength and frequency therefore equals the wave velocity:

Thus, if the wave in our earlier example had a wavelength of 0.5 m, its velocity is Wavelength and wave frequency are inversely related—doubling one halves the other.

CONCEPT CHECK
What is a wave? What four basic properties describe a wave, and what relationships, if any, exist among them?