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Direction of Wave Motion

FIG. 1.8 Electromagnetic waves All forms of radiation consist of electric and magnetic fields that oscillate at right angles to each other and to the direction of travel. They move through empty space at the velocity of light. The separation between adjacent wave crests is called the wavelength of the radiation and is often designated by the lower case Greek letter lambda or

X-rays Visible

Gamma rays ljhraviolei j Infrared Radio

X-rays Visible

Gamma rays ljhraviolei j Infrared Radio

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Wavelength (m)

FIG. 1.9 Electromagnetic spectrum Radiation from the Sun and other cosmic objects is emitted at wavelengths from less than 10~12 meters to greater than 104 meters. The visible spectrum is a very small portion of the entire range of wavelengths. The lighter the shading, the greater the transparency of the Earth's atmosphere. Solar radiation only penetrates to the Earth's surface at visible and radio wavelengths, respectively denoted by the narrow and broad white areas. Electromagnetic radiation at short X-ray and ultraviolet wavelengths, represented by the dark areas at the left, is absorbed in our air, so the Sun is now observed in these spectral regions from above the atmosphere in Earth-orbiting satellites.

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Wavelength (m)

FIG. 1.9 Electromagnetic spectrum Radiation from the Sun and other cosmic objects is emitted at wavelengths from less than 10~12 meters to greater than 104 meters. The visible spectrum is a very small portion of the entire range of wavelengths. The lighter the shading, the greater the transparency of the Earth's atmosphere. Solar radiation only penetrates to the Earth's surface at visible and radio wavelengths, respectively denoted by the narrow and broad white areas. Electromagnetic radiation at short X-ray and ultraviolet wavelengths, represented by the dark areas at the left, is absorbed in our air, so the Sun is now observed in these spectral regions from above the atmosphere in Earth-orbiting satellites.

in our atmosphere, never reaching the ground, tte wavelength of ultraviolet radiation, which is also absorbed in our air, is just a bit longer, between 10~8 and 10~7 meters, with extreme ultraviolet radiation lying in the short wavelength part of this range. In contrast radio waves are between 0.001 and 30 meters long. So radio waves can be as big as you are tall, or even as large as a house or skyscraper, too long to enter the eye and not energetic enough to affect vision.

Just as a source of sound can vary in pitch, or wavelength, depending on its motion, the wavelength of electromagnetic radiation shifts when the object emitting or reflecting the radiation moves with respect to the observer (Fig. 1.10). ttis is called the Doppler effect, after the Austrian physicist Christian Johann Doppler (1803-1853), who discovered it in 1842. If the motion is toward the observer, the Doppler effect

FIG. 1.10 Dopplereffect Astationary star (top) emits regularly spaced light waves that get stretched out or scrunched up if the star moves (bottom). Here we show a star moving away (bottom right) from the observer (bottom left). The stretching of light waves that occurs when the star moves away from an observer along the line of sight is called a redshift, because red light waves are relatively long visible light waves; the compression of light waves that occurs when the star moves along the line of sight toward an observer is called a blueshift, because blue light waves are relatively short. The wavelength change, from the stationary to moving condition, is called the Doppler shift, and its size provides a measurement of radial velocity, or the velocity of the component of the star's motion along the line of sight. The Doppler effect is named after the Austrian physicist Christian Doppler (1803-1853), who first considered it in 1842.

shifts the radiation to shorter wavelengths, and when the motion is away the wavelength becomes longer. Such an effect is responsible for the change in the pitch of a passing ambulance siren. Because everything in the Universe moves, the Doppler effect is a very important tool for astronomers; it measures the velocity of that motion along the line of sight to the observer.

Sometimes radiation is described by its frequency instead of its wavelength. Radio stations are, for example, denoted by their call letters and the frequency of their broadcasts, usually in units of a million cycles per second, or megahertz, for FM broadcasts.

tte frequency of a wave is the number of crests passing a stationary observer each second; the frequency therefore tells us how fast the radiation oscillates, or moves up and down, tte product of wavelength and frequency equals the velocity of light, so when the wavelength increases the frequency decreases and vice versa.

When light is absorbed or emitted by atoms, it behaves like packages of energy, called photons, which can be created or destroyed, tte photons are produced whenever a material object emits electromagnetic radiation, and they are consumed when matter absorbs radiation. Radiation therefore disappears and ceases to exist when absorbed by matter. But energy is neither created nor destroyed; it is just removed from the radiation.

Moreover, each elemental atom can only absorb and emit photon energy in specific amounts, ttis is a consequence of the unique arrangement of electrons in each atom, and the pattern of photon energy emitted and absorbed can therefore be used to identify the atom.

At the atomic level, the natural unit of energy is the electron volt, abbreviated eV. One electron volt is the energy an electron gains when it passes across the terminals of a 1-volt battery. A photon of visible light has an energy of about two electron volts, or 2 eV. Much higher energies are associated with nuclear processes; they are often specified in units of millions of electron volts, or MeV for short. A somewhat lower unit of energy is a thousand electron volts, called the kilo-electron volt or keV; it is often used to describe X-ray radiation.

tte interaction of each type of radiation with matter depends on the energy of its photons, and from the standpoint of the astrophysicist this is the most important property distinguishing one type of radiation from another. In fact, astronomers often describe energetic radiation, such as X-rays or gamma rays, in terms of its energy rather than its wavelength or frequency.

Photon energy is inversely proportional to the wavelength and directly proportional to the frequency. Radiation with a shorter wavelength or a higher frequency therefore corresponds to photons with higher energy. Radio photons have relatively long wavelengths and low frequencies, so they have less energy than the short-wavelength, high frequency X-ray radiation. The low energies of radio photons cannot easily excite the atoms of our atmosphere, so these photons easily pass through the air, even in stormy weather. In contrast, X-rays are totally absorbed when traveling just a short distance through the atmosphere. The energetic X-rays produced by machines here on Earth pass right through your skin, muscles, or teeth. It also takes much less energy to broadcast a radio signal over short distances than to take an X-ray of a broken bone.

tte energy of stellar radiation at a given wavelength can be related to the thermal energy, or the temperature, of the emitting gas. Hot stars tend to be bluer in color, for example, and colder stars are redder, ttis is because the most intense emission occurs at a radiation frequency and energy that increase with the temperature of the star's visible disk. In other words, the emitted power peaks out at a wavelength that varies inversely with the temperature, and this applies to all gaseous objects in the Cosmos, ttus, the cold, dark spaces between the stars radiate most intensely at the longer, invisible radio wavelengths, while a hot, million-degree gas emits most of its energy at short X-ray wavelengths that are also invisible.

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