Electromagnetic radiation travels though the vacuum of space in waves, which we shall also examine in some detail in Chapter 7. A wave—think of a water wave—is not a physical object, but a pattern of up-and-down or back-and-forth motion created by a disturbance. Waves are familiar to anyone who has thrown a rock in a pond of still water or watched raindrops striking a puddle. The wave pattern in the water, a series of concentric circles, radiates from the source of the energy, the impact of the rock or the rain drop. If anything happens to be floating on the surface of the water— say a leaf—the waves will transfer some of the energy of the splash to the leaf and cause it to oscillate up and down. The important thing to remember about waves is that they convey both energy and information. Even if we didn't actually see the rock or the raindrop hit the water, we would be able to surmise from the action of the waves that something had disturbed the surface of the water at a particular point.
A familiar example of waves propagated by the energy of a tossed stone.
(Image from the authors' collection)
Wavelength is the distance between two adjacent wave crests (high points) or troughs (low points). This distance is usually measured in meters or multiples thereof. Water waves may have wavelengths of a few meters, radio waves of a few centimeters, and the wavelengths of optical light are very short (~0.0000005 meters). Frequency is the number of wave crests that pass a given point per unit of time. By convention, frequency is measured in hertz. 1 Hz is equivalent to one crest-to-crest cycle per second and named in honor of the nineteenth-century German physicist Heinrich Rudolf Hertz.
The type of energy and information created and conveyed by electromagnetic radiation is more complex than that created and conveyed by the waves generated by a splash in the water. We will wait until Chapter 7 for a little lesson in wave anatomy, but do take a moment now to make sure that you understand two properties of waves: wavelength and frequency. Wavelength is the distance between two adjacent wave crests (high points) or troughs (low points), measured in meters. Frequency is the number of wave crests that pass a given point per unit of time (and has units of 1/second).
We think of the light from our reading lamp as very different from the x-rays our dentist uses to diagnose an ailing tooth, but both are types of electromagnetic waves, and the only difference between them is their wavelengths. Frequency and wavelength of a wave are inversely proportional to one another, meaning that if one of them gets bigger, the other one must get smaller. The particular wavelength produced by a given energy source (a star's photosphere, a planetary atmosphere) determines whether the electromagnetic radiation produced by that source is detected at radio, infrared, visible, ultraviolet, x-ray, or gamma ray wavelengths.
The waves that produce what we perceive as visible light have wavelengths of between 400 and 700
nanometers (a nanometer is 0.000000001 meter, or 1 x 10-9 m) and frequencies of somewhat less than 1015 Hz. Light waves, like the other forms of electromagnetic radiation, are produced by the change in the energy state of an atom or molecule. These waves, in turn, transmit energy from one place in the universe to another. The special nerves in the retinas of our eyes, the emulsion on photographic film, and the pixels of a CCD (Charge Coupled Device) electronic detector are all stimulated (energized) by the energy transmitted by waves of what we call visible light. That is why we "see."
The outer layers of a star consist of extremely hot gas. This gas is radiating away some fraction of the huge amounts of energy that a star generates in its core through nuclear fusion (see Chapter 16, "Our Star"). That energy is emitted at some level in all portions of the electromagnetic spectrum, so that when we look at a distant or nearby star (the sun) with our eyes, we are receiving a small portion of that energy.
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