Electromagnetic Fields Help (page 3)

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

Solar Observation

The Sun has been observed by radar. Even though the surface is not solid, the outward motion of subatomic particles has been detected: the solar wind . The “surface” of the Sun is different in the radio part of the EM spectrum as compared with the visible-light portion.

The Sun has, of course, no solid surface as does the Earth, Venus, or Mercury. The apparent diameter of the solar globe depends on the EM wavelength at which the Sun is observed. This makes it possible to examine the motion of the gases at different levels. Great turbulence exists there; we know this because Doppler shifts are constantly observed. Radar telescopes allow astronomers to calculate how fast the gases rise and fall as the Sun’s surface boils in an endless storm.

Effective Range

The maximum range of a radar telescope is limited by two factors. First, there is path loss , caused by the sheer physical distances over which radar signals must travel on their way from the antenna to the target and back. Second, the free-space speed of EM-wave propagation is finite. Although 299,792 km/s (186,282 mi/s) seems fast on a terrestrial scale, it is sluggish with respect to the Cosmos.

Path loss increases with distance and mandates more sensitive receivers and more powerful transmitters as the distance gets greater. There is a practical limit to how sensitive any rf receiver can be made. There is also a limit to how much power can be generated in a radar transmitter and a limit to how much gain can be realized with an antenna of manageable size. There is yet room for engineers to design and build larger antennas, more sensitive receivers, and more powerful transmitters. Eventually, however, economic considerations must prevail over scientific curiosity.

The propagation-speed constraint is insurmountable no matter what the size of the hardware budget. An echo from Pluto returns to Earth approximately 10 hours after the signal is sent. An echo from the nearest star system would not return for almost 9 years. Most of the star systems in our galaxy are so far away that the echoes from a radar set will not come back until many human lifetimes have passed. Earth-based radar astronomy will never be useful in the study of objects outside the Solar System.

Infrared Astronomy

EM energy at radio frequency has much greater wavelengths than energy in the visible part of the spectrum. The shortest radio microwaves measure approximately 1 mm in length; the reddest visible light has a wavelength of a little less than 0.001 mm. This is a span of a thousandfold, or three mathematical orders of magnitude, and it is called the infrared (IR) spectrum . In terms of frequency, IR lies below red visible light. It is from this fact that IR gets its name: the prefix infra- means “below” or “under.” Our bodies sense infrared as radiant warmth or heat. The IR rays are not literally heat, but they produce heat when they strike an absorptive surface such as human skin.

Observational Equipment

Stars, galaxies, planets, and other things in the Cosmos radiate at all wavelengths, not only at wavelengths convenient for humans to observe. In some portions of the IR spectrum, the atmosphere of our planet is opaque. Between about 770 nm (the longest visible red wavelength) and 2 micrometers (µm), our atmosphere is reasonably clear, and it is possible to observe IR energy in this wavelength range from surface-based locations. To see celestial images at longer IR wavelengths, the observations must be made from high in the atmosphere or from space.

The Moon, the Sun, and the planets all have been observed in IR, as have some stars and galaxies. IR observing equipment resembles optical apparatus. This is true of telescopes as well as cameras. Similar lenses, films, and sensors are used, and excellent resolution can be obtained. Special kinds of film have been developed recently, making observation possible at longer and longer IR wavelengths.

Stars In Ir

IR astronomy has helped scientists to discover certain peculiar dim stars that seem to radiate most of their energy in the IR range. Visually, such stars appear red and dim. However, like a faintly glowing electric-stove burner, they are powerful sources of IR. These stars have relatively low surface temperatures compared with other stars. At first thought, this seems to be a paradox, but the peak wavelength at which an object radiates is a direct function of the temperature. “Cool” stars produce radiation at predominantly longer wavelengths than “hot” stars. The hottest stars are comparatively weak radiators of IR. When astronomers talk about temperatures of celestial objects, they usually refer to the spectral temperature , which is determined by examining the EM radiation intensity from the object at various wavelengths.

IR astronomy is important in the study of evolving stars and star systems. As a cloud of interstellar dust and gas contracts, it begins to heat up, and its peak radiation wavelength becomes shorter and shorter. A cool, diffuse cloud radiates most of its energy in the radio part of the EM spectrum. Hot stars radiate largely in the UV and x-ray regions. Sometime between the initial contraction of the nebula and the birth of the star, the peak emission wavelength passes through the IR. The observation of IR is also important in the analysis of dying stars. As a white dwarf cools down and becomes a black dwarf, its peak radiation wavelength decreases. On its way toward ultimate cold demise, the star emits, for a certain period of time, most of its energy in the IR.

Measuring Temperature

The characteristic EM radiation from a star is a form of emission known as blackbody radiation . A blackbody is a theoretically perfect absorber and radiator of EM energy at all wavelengths. Any object having a temperature above absolute zero (–273°C or –459°F) has a characteristic pattern of wavelength emissions that depends directly on the temperature. For any EM-radiating object (and this includes everything in the Universe), the emission strength is maximum at a certain defined wavelength and decreases at longer and shorter wavelengths. If EM intensity is graphed as a function of wavelength or frequency, the result is a curve that resembles a statistical distribution (Fig. 18-6).

Observing the Invisible Infrared Astronomy Measuring Temperature

Figure 18-6. The graph of EM intensity radiated from a blackbody, as a function of frequency or wavelength, has a characteristic shape.

By observing an object at many different wavelengths including the radio region, the IR, the visible range, and the UV and x-ray spectra, the point of maximum emission can be found. Sometimes it can be inferred even if observations are not actually made at that wavelength by plotting points on an intensity-versus-wavelength graph and connecting the points with a smooth curve. From the maximum-emission wavelength, the temperature of the object can be estimated based on the assumption that it behaves as a blackbody. Figure 18-7 is a rough graph, on a logarithmic scale, of the function employed by scientists for this purpose.

Observing the Invisible Ultraviolet And Beyond

Figure 18-7. Temperature, in degrees Kelvin (absolute), as a function of the maximum-amplitude wavelength of a blackbody.

Ultraviolet And Beyond

As the wavelength of an EM disturbance becomes shorter than that of visible violet light, the energy contained in each individual photon increases. The UV range of wavelengths starts at about 390 nm and extends down to approximately 1 nm. The x-ray range extends from roughly 1 nm down to 0.01 nm. (The precise dividing line between the shortest UV and the longest x-ray wavelengths depends on whom you ask.) The gamma-ray spectrum consists of EM waves shorter than 0.01 nm. Cosmic radiation is entirely different in origin; it arises from high-speed subatomic particles thrown off by the most energetic celestial objects.

The Uv Atmospheric “window”

As the wavelength decreases, the atmosphere of Earth becomes highly absorptive at about 290 nm. At still shorter wavelengths, the air is essentially opaque. (This is a good thing because it protects the environment against damaging UV radiation from the Sun.) The atmosphere scatters some EM radiation even in the visible blue and violet parts of the spectrum. This is why the sky appears blue to our eyes. Ground-based observatories can see something of space at wavelengths somewhat shorter than the visible violet, but when the wavelength gets down to 290 nm, nothing more can be seen. At the shortest UV wavelengths ( hard UV and extreme UV ), as in the case of the far IR, it is necessary to place observation apparatus above the atmosphere.

“Seeing” Uv

Glass is virtually opaque to UV, so ordinary cameras with glass lenses cannot be used to take conventional photographs in this part of the spectrum. Instead, a pinhole-type device is used, and this severely limits the amount of energy that passes into the detector. While a camera lens has a diameter of several centimeters, a pinhole is less than a millimeter across. This does not present a problem for photographing the Sun or the Moon, but for other celestial objects it is not satisfactory.

For analysis of fainter celestial objects in UV, an instrument called a spectrophotometer is used. This device is a sophisticated extended-range spectrometer in which a diffraction grating (not a glass prism) is used to disperse EM energy into its constituent wavelengths. By moving the sensing device back and forth, any desired wavelength can be singled out for observation, even those in the IR or UV spectrum. The principle of the spectrophotometer is shown in Fig. 18-8. In the long-wavelength or soft-UV range, a photoelectric cell can serve as a sensor. In the hard- and extreme-UV spectra, radiation counters are sometimes used, similar to the apparatus employed for the detection and measurement of x-rays and gamma rays. For photographic purposes, ordinary camera film works in the soft-UV range, but special film, rather like x-ray film, is used to photograph hard-UV and extreme-UV images.

Observing the Invisible Ultraviolet And Beyond “seeing” Uv

Figure 18-8. Functional diagram of a spectrophotometer, which can be used to sense and measure UV radiation.

Sources Of Uv

The hot type O and B stars are strong sources of UV radiation. These stars evidently radiate more energy in the UV than in any other part of the EM spectrum. Type O and B stars generally are young stars. Within the visible part of the spectrum, their energy tends to be concentrated at the shortest wavelengths, so such stars look blue to us. The surface temperatures of these stars are much greater than the temperature at the surface of our Sun, which is a type G star. Type O and B stars have surface temperatures ranging from about 15,000 to 25,000°C (27,000 to 45,000°F).

Type A and F stars radiate smaller amounts of UV energy than type O and B stars. Most type A and F stars look white. Still, these stars are hotter than the Sun. Temperatures at their surfaces range from about 8,000 to 15,000°C (14,000 to 27,000°F).

Type K and M stars have the coolest surfaces of any stars, ranging down to about 1,500°C (2,700°F). The greatest amount of visible radiation from such stars falls into the wavelengths corresponding to red and orange. These stars produce comparatively little UV radiation.

Supernovae produce fantastic amounts of visible light, but they also emit large amounts of UV. The explosion of a supernova within a few light-years of the Solar System would be a spectacular sight; the brilliance of the star would exceed that of the full Moon. However, the UV radiation, despite the distance, would compare with that from our own Sun. You could get a “star-burn” from the supernova, even at night, and certainly without realizing it, because the IR intensity would be relatively low. The shortest UV rays, which would cause the most damage to life on our planet, would be largely kept at bay by the atmosphere.

A supernova, after having thrown off a cloud of gas in the process of exploding, causes ionization of the cloud because of UV radiation. This causes the gas to fluoresce, or glow visibly. Such a glowing cloud can be seen from many light-years away. Sometimes nearby stars, of the hot types O and B, cause ionization of interstellar gas. This produces such spectacular astronomical objects as the Great Nebula in Orion, the Horsehead Nebula, and others. Bright emission nebulae betray the presence of UV sources in their vicinity. A fluorescent lightbulb operates on the same principle as the emission nebulae; the coating on the inside of such a bulb is set to glow by UV radiation from the gases within.

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