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#1
October 12th 03, 04:19 PM
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Up and down the spectrum
I'd like to check my understanding of these factoids about the
EM spectrum. In addition to correcting my errors, what other characteristics distinguish the various parts of the spectrum? Radio waves are characterized by their ability to be emitted and detected by antennae. Metals reflect them; other materials are transparent to them. Microwaves are distinguished from other radio waves by their ability to be channeled by waveguides. Electronic components used to generate and detect microwaves are about the same size as the waves themselves. Infrared is distinguished from radio and microwaves by its ability to interact with individual electrons in atoms, the photoelectric effect, and its ability to be optically lensed. Infrared is absorbed by most materials. Visible light is distinguished from infrared and ultraviolet by its visibility to humans. (I'm sure I got this one right!) Ultraviolet is ionizing. (Starting at what frequencies in what materials?) UV is absorbed by most materials. X-Ray is distinguished from ultraviolet by its ability to penetrate materials, roughly in inverse proportion to the material's density, and its inability to be optically lensed. Gamma is distinguished from X-ray by its ability to interact with nucleons. (Do gammas usually whizz right past electrons in atoms without interacting? Or do they interact with electrons just as often as lower-energy X-rays do?) -- Jeff, in Minneapolis Subtract 1 from my e-mail address above for my real address. .. 
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#2
October 12th 03, 06:07 PM
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Dear Jeff Root:
"Jeff Root" wrote in message om... I'd like to check my understanding of these factoids about the EM spectrum. In addition to correcting my errors, what other characteristics distinguish the various parts of the spectrum? Radio waves are characterized by their ability to be emitted and detected by antennae. antennae - devices made of conductors (such as metals) Metals CONDUCTORS reflect them; other materials are transparent to them. translucent might be a better term. There are always losses. I suppose it doesn't matter, since a color change is not inherent, and translucent can imply a color change. Microwaves are distinguished from other radio waves by their ability to be channeled by waveguides. The EM spectrum is assigned by wavelength as standard description. The above sentence would be better stated "Microwaves are distinguished from other light..." Electronic components used to generate and detect microwaves are about the same size as the waves themselves. Look at the physical size of a Klystron tube. Much larger than microwaves. Also reflected by conductors (microwave antenna dishes, for example) Infrared is distinguished from radio and microwaves by its ability to interact with individual electrons in atoms, the photoelectric effect, and its ability to be optically lensed. Infrared is absorbed by most materials. You can lens any light, if you can establish a medium with a different propagation speed. You can lens microwaves, and radio waves. It might take miles of material to do it with radio waves, but you could do it. Visible light is distinguished from infrared and ultraviolet by its visibility to humans. (I'm sure I got this one right!) Ultraviolet is ionizing. (Starting at what frequencies in what materials?) UV is absorbed by most materials. The frequency (or wavelength) thresholds are established by convention. X-Ray is distinguished from ultraviolet by its ability to penetrate materials, roughly in inverse proportion to the material's density, and its inability to be optically lensed. X-rays can be lensed. At some point lensing is just an engineering problem. The more dense a material is, the more likely it is to absorb X-rays. Gamma is distinguished from X-ray by its ability to interact with nucleons. (Do gammas usually whizz right past electrons in atoms without interacting? Or do they interact with electrons just as often as lower-energy X-rays do?) Here is where a number of sources will conflict. Gamma is commonly reserved for light that is emitted from a nuclear decay. In this context it can be inclusive of what others will call X-rays (in energy). Personally, I like it if we start out with cutoffs based on wavelength, that we stick to it. Gamma is ionizing radiation. The common physical sources are less intense (in the number of photons emitted) than the average candle. To the gamma photons emitted from a lump of Co-60, we are like smoke. And thin smoke at that. David A. Smith
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#3
October 12th 03, 07:15 PM
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#4
October 13th 03, 07:47 AM
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David A. Smith replied to Jeff Root:
Radio waves are characterized by their ability to be emitted and detected by antennae. antennae - devices made of conductors (such as metals) Do you know of any antenna made of a conductive material other than metal? Can you provide a link to a description of such an antenna? Microwaves are distinguished from other radio waves by their ability to be channeled by waveguides. The EM spectrum is assigned by wavelength as standard description. That has nothing to do with the info I'm looking for. The above sentence would be better stated "Microwaves are distinguished from other light..." How is that better? I worded it as I did to indicate that microwaves are included in the "radio" part of the spectrum, but have characteristics that distinguish them from other radio waves. My understanding is that microwaves and higher frequencies can be channeled by waveguides, while radio waves of lower frequencies cannot. My understanding may be way off. Do the Earth and ionosphere form a waveguide for shortwave radio? Is that essentially the same phenomenon? If so, then what *does* distinguish microwaves from other radio waves? Electronic components used to generate and detect microwaves are about the same size as the waves themselves. Look at the physical size of a Klystron tube. Much larger than microwaves. It appears that any assertion on the Internet generates its antithesis. I included the info about the size of components because of this reply to a similar question last December: | What distinguishes radio waves from microwaves, and microwaves | from infrared? | | Radio waves which are not microwaves are handled with normal | electronics, i.e. components which are much smaller than the | wavelength. | | Infrared waves are handled with optics, i.e. components which | are much larger than the wavelength. | | Microwaves are handled with microwave electronics, i.e. components | which are of approximately the same size as the wavelength. Do you disagree with that? Infrared is distinguished from radio and microwaves by its ability to interact with individual electrons in atoms, the photoelectric effect, and its ability to be optically lensed. Infrared is absorbed by most materials. You can lens any light, if you can establish a medium with a different propagation speed. You can lens microwaves, and radio waves. It might take miles of material to do it with radio waves, but you could do it. Ah. I asked about that last year but got no answer. Ultraviolet is ionizing. (Starting at what frequencies in what materials?) UV is absorbed by most materials. The frequency (or wavelength) thresholds are established by convention. That makes no sense to me. Shine a low-intensity monochromatic light on some material. Depending on the material, if the light has a high enough frequency, the material will ionize. If the frequency is not that high, it won't. No conventions involved. X-Ray is distinguished from ultraviolet by its ability to penetrate materials, roughly in inverse proportion to the material's density, and its inability to be optically lensed. X-rays can be lensed. Can you provide a source for that bit of info? An earlier answer to the question: | What distinguishes X-rays from ultraviolet? | | UV can be reflected and refracted using optics (although special | optics of course); X-rays no longer can. | | Many years after this distinction was made, it was found that X-rays | _can_ be reflected under some very special circumstances: during | grazing incidence i.e. when the incident ray is very nearly parallell | to the reflecting surface. By using this principle the first X-ray | telescopes (the Wollter telescopes) were constructed in the 1970's. By "optically lensed" I meant "refracted", and deliberately left out mention of reflection. Gamma is distinguished from X-ray by its ability to interact with nucleons. (Do gammas usually whizz right past electrons in atoms without interacting? Or do they interact with electrons just as often as lower-energy X-rays do?) Here is where a number of sources will conflict. Gamma is commonly reserved for light that is emitted from a nuclear decay. In this context it can be inclusive of what others will call X-rays (in energy). Personally, I like it if we start out with cutoffs based on wavelength, that we stick to it. I don't need legal definitions. I don't need boundary lines between different parts of the spectrum-- I need characteristics which distinguish different parts of the spectrum. My impression is that *most* nuclear transitions which emit or absorb photons are at higher energy levels (higher frequency) than electron transitions in atoms. Electron transitions are generally caused by and emit IR through X-Ray, while nuclear transitions are generally caused by and emit higher frequencies. Is that right? Gamma is ionizing radiation. The common physical sources are less intense (in the number of photons emitted) than the average candle. To the gamma photons emitted from a lump of Co-60, we are like smoke. And thin smoke at that. Okay, that means the gammas go through us easily. But does it mean that they knock electrons out of atoms all along the path as they pass through, or do they just pass through with usually no effect at all, but are stopped when they hit a nucleus? -- Jeff, in Minneapolis Subtract 1 from my e-mail address above for my real address. ..
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#5
October 13th 03, 07:49 AM
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Alan Moore replied to Jeff Root:
Radio waves are characterized by their ability to be emitted and detected by antennae. Metals reflect them; other materials are transparent to them. Well, not exactly. Any change in dielectric properties can cause a reflection. Radar at sufficiently high frequencies will detect animals, and even at moderate frequencies, flocks of birds or insects. Radar is in the microwave part of the spectrum. I traded some clarity for brevity, but the layout of my list should have made it fairly obvious that I was referring mainly to the part of the radio spectrum with frequencies lower than microwaves. Do the qualifications apply below the microwave region? Microwaves are distinguished from other radio waves by their ability to be channeled by waveguides. Electronic components used to generate and detect microwaves are about the same size as the waves themselves. This is also true for other radio waves. Just keep in mind the necessary relationship between waveguide dimension and signal wavelength, and you'll see that there comes a point at which waveguides become inconveniently large. Do I understand you to be saying that all radio waves could be handled by waveguides and other components of about the same size as the waves, but that below the microwave region, those waveguides and other components become impractical because of their size, and so are generally not used? Does it mean that the distinction between microwaves and the rest of the radio portion of the spectrum is in the practical choice of components actually used for generating and detecting the waves? Ultraviolet is ionizing. (Starting at what frequencies in what materials?) UV is absorbed by most materials. You can look up ionization potentials for elements in standard physical and chemical handbooks. Actually this starts in the infra-red. Otherwise vacuum tubes wouldn't work, as they rely on those energy levels to get electrons away from the solid cathode and into the vacuum where they can be accelerated away from it. Are we talking about the same thing? The cathode in a tube needs to be hot. I'd say ionization by EM radiation is when that EM radiation falls on a material and ionizes it, without a need for the material to be particularly hot. Am I wrong? X-Ray is distinguished from ultraviolet by its ability to penetrate materials, roughly in inverse proportion to the material's density, and its inability to be optically lensed. Yes, sort of. Whereas lower energy photons may result in ionization of atoms by ejecting an electron from an outer shell, x-rays may eject electrons from inner shells. Why only "sort of"? It looks to me like you added a detail of how and where the ionization takes place, but didn't change anything I said, at all. Gamma is distinguished from X-ray by its ability to interact with nucleons. (Do gammas usually whizz right past electrons in atoms without interacting? Or do they interact with electrons just as often as lower-energy X-rays do?) As often, or oftener. This is why they are grouped with the "soft" x-rays and UV as ionizing radiation. I'm guessing that you said '"soft" x-rays' rather than simply "x-rays" because hard x-rays are the same as gamma rays. Is that right? So if I shine a million soft x-ray photons at my left hand, and a million gamma rays at my right hand, not only will my right hand be hit harder by each ray than my left hand is, but more electrons will be hit and knocked out of their atoms than will happen in my left hand? -- Jeff, in Minneapolis Subtract 1 from my e-mail address above for my real address. ..
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#6
October 13th 03, 06:45 PM
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In message , Jeff Root
writes David A. Smith replied to Jeff Root: Radio waves are characterized by their ability to be emitted and detected by antennae. antennae - devices made of conductors (such as metals) Do you know of any antenna made of a conductive material other than metal? Oddly enough, yes, as this is an astronomy group. There's that radio interferometer that was mounted on a cliff and used the sea as one element. -- "It is written in mathematical language" Remove spam and invalid from address to reply.
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#7
October 13th 03, 07:28 PM
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In article ,
Jeff Root wrote: David A. Smith replied to Jeff Root: Radio waves are characterized by their ability to be emitted and detected by antennae. antennae - devices made of conductors (such as metals) Do you know of any antenna made of a conductive material other than metal? Can you provide a link to a description of such an antenna? E.g. rods of carbon ought to work too..... Microwaves are distinguished from other radio waves by their ability to be channeled by waveguides. The EM spectrum is assigned by wavelength as standard description. That has nothing to do with the info I'm looking for. The above sentence would be better stated "Microwaves are distinguished from other light..." How is that better? I worded it as I did to indicate that microwaves are included in the "radio" part of the spectrum, but have characteristics that distinguish them from other radio waves. My understanding is that microwaves and higher frequencies can be channeled by waveguides, while radio waves of lower frequencies cannot. My understanding may be way off. Do the Earth and ionosphere form a waveguide for shortwave radio? Not for shortwave radio, but for longwave radio! Shortwave radio work by having the radio waves reflected in the electron layers in the ionosphere. An EM radiation can be channeled by waveguides, however the cross section of the waveguide must have a size comparable to the wavelength of the EM radiation to be guided. So you can build a waveguide for shortwave, but it would be quite large.... Is that essentially the same phenomenon? If so, then what *does* distinguish microwaves from other radio waves? The distinction was originally defined something like this: Normal radio waves: the wavelength is much longer than the size of the individual components. Microwaves: the wavelength is of a size comparable to (= of the same order of magnitude as) the size of the individual components. Optics: the wavelength is much shorter than the size of the individual components. This was made at a time before the transistor was invented, and thus the vacuum electron tube was the only electronic component available which provided amplification. Since then the electronics has been miniaturized, and thus it's today feasible to, in the longer wavelengths parts of the traditional microwave band, use discrete components. But the definition of "microwaves" remains approximately the same as in the 1930-40's. And there are no precise limits defined for "microwaves"; different sources give somewhat different wavelength limits. Electronic components used to generate and detect microwaves are about the same size as the waves themselves. Look at the physical size of a Klystron tube. Much larger than microwaves. It appears that any assertion on the Internet generates its antithesis. I included the info about the size of components because of this reply to a similar question last December: | What distinguishes radio waves from microwaves, and microwaves | from infrared? | | Radio waves which are not microwaves are handled with normal | electronics, i.e. components which are much smaller than the | wavelength. | | Infrared waves are handled with optics, i.e. components which | are much larger than the wavelength. | | Microwaves are handled with microwave electronics, i.e. components | which are of approximately the same size as the wavelength. Do you disagree with that? Infrared is distinguished from radio and microwaves by its ability to interact with individual electrons in atoms, the photoelectric effect, and its ability to be optically lensed. Infrared is absorbed by most materials. You can lens any light, if you can establish a medium with a different propagation speed. You can lens microwaves, and radio waves. It might take miles of material to do it with radio waves, but you could do it. Ah. I asked about that last year but got no answer. Ultraviolet is ionizing. (Starting at what frequencies in what materials?) UV is absorbed by most materials. The frequency (or wavelength) thresholds are established by convention. That makes no sense to me. Shine a low-intensity monochromatic light on some material. Depending on the material, if the light has a high enough frequency, the material will ionize. If the frequency is not that high, it won't. No conventions involved. However the minimum frequency which will cause ionization varies a lot between different materials. Nah, the limit between UV and visible light is instead determined by what the human eye can see. -- ---------------------------------------------------------------- Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN e-mail: pausch at stockholm dot bostream dot se WWW: http://www.stjarnhimlen.se/ http://home.tiscali.se/pausch/
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#8
October 14th 03, 12:00 AM
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Jonathan Silverlight wrote:
In message , Jeff Root writes David A. Smith replied to Jeff Root: Radio waves are characterized by their ability to be emitted and detected by antennae. antennae - devices made of conductors (such as metals) Do you know of any antenna made of a conductive material other than metal? Oddly enough, yes, as this is an astronomy group. There's that radio interferometer that was mounted on a cliff and used the sea as one element. Well, the sea was used as a reflector. The energy was still collected by a more conventional antenna.
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#9
October 15th 03, 03:06 AM
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#10
October 15th 03, 08:49 AM
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Alan Moore replied to Jeff Root:
Radio waves are characterized by their ability to be emitted and detected by antennae. Metals reflect them; other materials are transparent to them. Well, not exactly. Any change in dielectric properties can cause a reflection. Radar at sufficiently high frequencies will detect animals, and even at moderate frequencies, flocks of birds or insects. Radar is in the microwave part of the spectrum. I traded some clarity for brevity, but the layout of my list should have made it fairly obvious that I was referring mainly to the part of the radio spectrum with frequencies lower than microwaves. Do the qualifications apply below the microwave region? Oh, yes. Even quite long wavelength signals are reflected by the rocky surfaces of the moon and planets, although under some circumstances, they may penetrate to some depth. Rocks aren't exactly metal-free. I tend to think of silicon as being rather metallic. But perhaps the silicon in quartz or the calcium in marble or even the iron, magnesium, and titanium in basalt lose their relevant metallic properties when compounded? Does it mean that the distinction between microwaves and the rest of the radio portion of the spectrum is in the practical choice of components actually used for generating and detecting the waves? There are other differences, of course. The ionosphere is absorbing at some wavelengths, reflecting at others, transparent at sufficiently high frequencies. The cutoffs vary depending on the degree of ionization present. Ionized material of any kind has metal-like properties, because of those free electrons... Ultraviolet is ionizing. (Starting at what frequencies in what materials?) UV is absorbed by most materials. You can look up ionization potentials for elements in standard physical and chemical handbooks. Actually this starts in the infra-red. Otherwise vacuum tubes wouldn't work, as they rely on those energy levels to get electrons away from the solid cathode and into the vacuum where they can be accelerated away from it. Are we talking about the same thing? The cathode in a tube needs to be hot. I'd say ionization by EM radiation is when that EM radiation falls on a material and ionizes it, without a need for the material to be particularly hot. Am I wrong? I wouldn't say right or wrong. It takes less energy to remove an electron from an atom when that atom is already in an excited state, with the electron "boosted" into a higher orbital. So, given atoms or molecules in excited states, lower energy EM radiation will result in ionization than would be required at very low temperatures. Okay. Clearly I need to review the basics. Temperature is obviously on an equal footing with incident light as a factor in causing ionization. I need to see how the two factors work together, and not ignore one when I study the other. X-Ray is distinguished from ultraviolet by its ability to penetrate materials, roughly in inverse proportion to the material's density, and its inability to be optically lensed. Yes, sort of. Whereas lower energy photons may result in ionization of atoms by ejecting an electron from an outer shell, x-rays may eject electrons from inner shells. Why only "sort of"? It looks to me like you added a detail of how and where the ionization takes place, but didn't change anything I said, at all. Without knowing where the electron is being ejected from, there isn't any difference. I hadn't noticed your statement about refraction, but in fact, x-rays can be refracted. The significance of the ejection of electrons from inner shells is that there are secondary effects as further radiation is emitted as electrons from outer shells move into the vacated inner shell, emitting longer wavelength x-rays, UV and visible light. Hey, that's interesting! I'll be looking for support of the assertion that x-rays can be refracted, though. The consensus *seems* to be that they can't. So if I shine a million soft x-ray photons at my left hand, and a million gamma rays at my right hand, not only will my right hand be hit harder by each ray than my left hand is, but more electrons will be hit and knocked out of their atoms than will happen in my left hand? Yes. Generally speaking, higher energy photons will do more damage than lower energy photons. I think *everybody* knows that. I just want to confirm whether higher-energy photons (hard x-ray or gamma) are more likely to interact with electrons in atoms than lower-energy photons (UV or soft x-ray) are, or if they are less likely to interact, though either way they interact more energetically when they do. -- Jeff, in Minneapolis Subtract 1 from my e-mail address above for my real address. ..
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