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Electromagnetic Fields Help (page 4)

By — McGraw-Hill Professional
Updated on Sep 18, 2011

The Sun In Uv

The Sun’s surface temperature is in the neighborhood of 6,000°C (11,000°F). Our parent star emits some UV, but not a great deal of it as stars go. This is fortunate for the kind of life that has evolved on this planet. If the Sun were a hotter star, life on any Earthlike planet in its system would have developed in a different way, if at all.

The Sun’s UV radiation has been investigated using equipment aboard rockets and satellites. The UV surface of the Sun is somewhat above the visual surface. This tells us that as the altitude above the photosphere (visible surface) increases the temperature rises. If our eyes suddenly became responsive to a range of wavelengths only half as long as they actually are—say, a continuum of 200 to 400 nm—the disk of the Sun would seem a little larger than it appears in the visible range. We would ascribe to the Sun a different photosphere.

If our eyes suddenly became UV eyes, the Sun not only would look slightly larger in size but it also would appear less bright. The atmosphere of Earth transmits EM rays poorly in the short-wavelength part of the range 200 to 400 nm. Assuming that the longest detectable wavelengths appeared “red” to us, we would consider the Sun a ruddy star. Vision is a subjective thing. We would be equally impressed with the unnatural blueness and brilliance of the Sun if our eyes suddenly became responsive to, say, a wavelength range of 800 to 1,600 nm.

X-rays

The x-ray spectrum consists of EM energy at wavelengths from approximately 1 to 0.01 nm. This is 2 mathematical orders of magnitude. Proportionately, the x-ray spectrum is vast compared with the visible range.

As the wavelength of x-rays become shorter, it becomes increasingly difficult to direct and focus them. This is so because of the penetrating power of the short-wavelength rays. A piece of paper with a tiny hole can work for UV photography; in the x-ray spectrum, the radiation passes right through the paper. However, if x-rays encounter a reflecting surface at a nearly grazing angle, and if the reflecting surface is made of suitable material, some degree of focusing can be realized. The shorter the wavelength of the EM energy, the smaller the angle relative to the surface must be if reflection is to take place. At the shortest x-ray wavelengths, the angle must be smaller than 1 degree of arc. This grazing reflection effect is shown in Fig. 18-9 A . The focusing mirror is tapered in the shape of an elongated paraboloid. Figure 18-9 B is a rough illustration of how an x-ray telescope achieves its focusing. As parallel x-rays enter the aperture of the reflector, they strike its inner surface at a grazing angle. The x-rays are brought to a focal point, where a radiation counter or detector is placed.

Observing the Invisible Ultraviolet And Beyond X-rays

Figure 18-9. At A , x-rays are reflected from a surface only when they strike at a grazing angle. At B , a functional diagram of an x-ray focusing and observing device.

The resolving power of an x-ray telescope, such as the one shown in Fig. 18-9 B , is not as good as that obtainable with optical apparatus, but it does allow the observation of some celestial x-ray sources. As is the case with UV radiation, x-rays from space must be viewed from above the atmosphere of our planet; x-ray telescopes aboard rockets and satellites send their information back to Earth by radio.

Sources Of X-rays

After the development of high-altitude rockets and space vehicles, it became possible to look at the x-ray sky. Powerful x-ray sources were found, but there at first appeared to be no explanation for some of them. Even type O and B stars do not produce large amounts of radiation in the x-ray spectrum; they are not hot enough. The most interesting x-ray objects appeared to produce more energy in the x-ray region than at longer wavelengths.

Some tentative hypotheses have been brought forward to explain intense, pointlike x-ray objects, which have, because of their apparent location within our galaxy, been called x-ray stars . They are not supernovae; their radiation wavelengths are too short even for that. Besides, they are not visually bright enough to be supernovae. X-ray stars are found more commonly than supernovae. Some astronomers theorize that the x-ray objects are binary stars in very close mutual orbits. Matter from one of the stars in a binary system could be torn away from the other member by gravitational forces. The stars might even be in mutual contact. The exchange of matter between two stars in such close association could account for the production of large amounts of x-rays.

Another suggestion concerning the nature of x-ray stars has been given: They are binary star systems in which one member is a neutron star or a very dense black dwarf. Still another theory holds that the strange stars are binary systems containing black holes. The gravitational influence of a neutron star or black hole is sufficient to account for the x-rays; as matter is torn from the visible member of such a binary system, the hotter interior layers are exposed, and this can produce radiation at very short wavelengths. The idea that matter is being ripped out of a star is supported by the existence of Doppler shifts in the x-rays.

Some x-ray objects seem to be outside of our galaxy. Certain quasars and radio galaxies have been associated with strong sources of x-rays. Some astronomers have hypothesized that interaction among the photons of radiant energy at different wavelengths, as they collide with each other, could be responsible for the x-ray emissions from extragalactic objects. Some galaxies and quasars apparently have regions of tremendously high temperature—hotter than anything we know in our Milky Way—and this state of affairs can generate highly energetic EM waves that peak in the x-ray portion of the EM spectrum.

Gamma Rays

As the wavelength of EM energy becomes shorter than the hardest x-rays, it becomes more and more difficult to obtain an image. The cutoff point where the x-ray region ends and the gamma-ray region begins is approximately 0.01 nm. Gamma rays can get shorter than this without limit. The gamma classification represents the most energetic of all EM fields. Short-wavelength gamma rays can penetrate several centimeters of solid lead or more than a meter of concrete. They are even more damaging to living tissue than x-rays. Gamma rays come from radioactive materials, both natural (such as radon) and human-made (such as plutonium).

Radiation counters are the primary means of detecting and observing sources of gamma rays. Gamma rays can dislodge particles from the nuclei of atoms they strike. These subatomic particles can be detected by a counter. One type of radiation counter consists of a thin wire strung within a sealed, cylindrical metal tube filled with certain gases. When a high-speed subatomic particle enters the tube, the gas is ionized for a moment, and conduction occurs between the inner wire and the cylinder. A voltage is applied between the wire and the outer cylinder so that a pulse of current occurs whenever the gas is ionized. This pulse produces a click in the output of an amplifier connected to the device.

A simplified diagram of a radiation counter is shown in Fig. 18-10. A glass window with a metal sliding door is cut in the cylinder. The door can be opened to let in particles of lower energy and closed to allow only the fastest particles to get inside. High-speed particles, which are tiny yet massive for their size, have no trouble penetrating the window glass if they are moving fast enough. Yet gamma rays can penetrate into the tube with ease, even when the door is closed.

Cosmic Particles

If you sit in a room with no radioactive materials present and switch on a radiation counter with the window of a tube closed, you’ll notice an occasional click from the device. Some of the particles come from the Earth; there are radioactive elements in the ground almost everywhere (usually in small quantities). Some of the radiation comes indirectly from space. These particles strike atoms in the atmosphere, and these atoms in turn eject other subatomic particles that arrive at the counter tube.

Observing the Invisible Ultraviolet And Beyond Cosmic Particles

Figure 18-10. Simplified diagram of a radiation counter.

The direction of arrival of high-speed atomic particles can be determined, to a certain extent, by means of a device called a cloud chamber . The air in a small enclosure can be treated especially to produce condensation when a subatomic particle enters, and the path of the particle will show up as a vapor trail.

In the early 1900s, physicists noticed radiation apparently coming from space. They found that the strange background radiation increased in intensity when observations were made at high altitude; the radiation level decreased when observations were taken from underground or underwater. This space radiation has been called secondary cosmic radiation or secondary cosmic particles . The actual particles from space, called primary cosmic particles , usually do not penetrate far into the atmosphere before they collide with and break up the nuclei of atoms. To observe primary cosmic particles, it is necessary to ascend to great heights, and as with the UV and x-ray investigations, this was not possible until the advent of the space rocket.

While the radiation in the EM spectrum—the radio waves, IR, visible light, UV, x-rays, and gamma rays—consists of photons traveling at the speed of light, cosmic particles are matter, traveling at speeds almost, but not quite, as fast as light. At such high speeds, the protons, neutrons, and other heavy particles gain mass because of relativistic effects, and this renders them almost immune to Earth’s magnetosphere. Such particles, arriving in the upper atmosphere, come to us in a nearly perfect straight-line path despite the magnetic field of our planet. By carefully observing the trails of the particles in a cloud chamber aboard a low-orbiting space ship, it is possible to ascertain the direction from which they have come. Over time, cosmic-particle maps of the heavens can be generated and compared with maps at various EM wavelengths.

Practice problems of this concept can be found at: Electromagnetic Fields Practice Problems

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