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When is a radio wave not a radio wave?
Radio wave redirects to Radio frequency, which is (according to the spectrum guide at the bottom of the page) the name for electromagnetic radiation with a far longer wavelength. Can someone more knowledgable than me please clarify or rephrase this appropriately? - Fourohfour 19:17, 21 September 2005 (UTC)
- The term 'radio wave' is used interchangably with 'radio frequency', to refer to frequency bands commonly used for radio transmission (or any other frequencies in that neighborhood). This would encompass everything from quite low frequencies (i.e., kilohertz) up to the microwave range (a few gigahertz or even up to 100 gigahertz, depending on who you ask). But everybody agrees that all of these frequencies are lower than 1 terahertz (since 1 terahertz = 1000 gigahertz = 1,000,000 megahertz). Since wavelength is the reciprocal of frequency (more precisely, wavelength * frequency = speed of light, which is constant), higher frequencies have shorter wavelengths. Thus, radio waves (lower frequencies) always have longer wavelengths than terahertz radiation. And, similarly, terahertz radiation has a longer wavelength than visible light, since the frequency is lower.
- Does that answer your question? - 22.214.171.124, at 23:25, 31 December 2005 (UTC)
is it really necessary to link back to the same page within the 10 first words? - 126.96.36.199, at 08:47, 7 April 2006
- Not at all. Rainwarrior 02:35, 2 May 2006 (UTC)
defining the subject
- I don't know what I could tell you that's not in the article lead. It's anything using very high frequency electromagnetic waves (particularly, ones in the Terahertz band). - Rainwarrior 15:19, 30 July 2006 (UTC)
Ok I get it but what's with this fuss about Terahertz 'Technology'; what differentiates it from usage of other waves in electromagnetic spectrum notfew years back INTEL anounced it is using some application of terherts tecnology in its processor so what does it differ from conventional electronic devices used such as diodes transistrors etc and can u tell me its other applications that is't mentioned here — Preceding unsigned comment added by Backar19 (talk • contribs) on 17:17, 31 July 2006 (UTC)
THz and tissue/water penetration.
The suggestion that THz technology can replace mammography and penetrate several cm of water 'at some THz frequencies' is simply wrong.
Anywhere above 100GHz the absorption depth in water is 100 microns or less. Transmission through cm of water of flesh yields huge attenuations, such tht here is no possibility, with any source or detector, of making a system; attenuations typically eceed 10^30!
Redefining <100GHz or 10um ('30THz') as terahertz technology is simply spurious; the first is routine microwave, the second mid infrared.
- Clarification: Since tissue is not 100% water, it can be possible to look through >1cm of certain tissue, if it contains relatively little water. But e.g. mammography will not work. --188.8.131.52 (talk) 17:55, 28 January 2008 (UTC)
THz versus millimeter and submilleter waves
One terahertz is Hz. Conventionally the microwave band extends to 30 GHz or so. While the far-IR is nominally reckoned to start at around 1 THz. So the terahertz band lies between micowaves and the far-IR. On the other hand, in this frequency range the wavelengths of electromagnetic waves (in vacuum) are millimeter or sub-millimeter. So, logically, terahertz waves are the same thing as millimeter or submillimeter waves. However, in practice people who use the term terahertz are generally speaking of signals generated by ultrafast optical techniques or far-IR lasers. Focusing a sub-picosecond pulse on a photoconductive antenna of suitable dimensions will produce EM waves in the THz band. On the other hand, people who use the term millimeter or submillimeter waves are invariably speaking of sources and detectors based on harmonic multiplication of microwave signals.
There have been commercial solid-state sources of millimeter and submillimeter waves for many years. AB Millimeter in Paris, for instance, produces a system that covers the entire range from 8 GHz to 1000 GHz with solid state sources and detectors. Nowadays, most time-domain work is done via ultrafast lasers. One of the key application of millimeter and submillimeter waves is the study of condensed matter in high magnetic fields since at high fields (say above 15 T), the Larmor frequencies are in the submillimeter band. —The preceding unsigned comment was added by 184.108.40.206 (talk • contribs) 04:22, 4 December 2006 (UTC).
- I added a discussion of the distinction between millimeter/submillimeter waves and terahertz waves. I also mentioned the use of solid state sources and detectors which have been around for many years. Finally I included the two biggest current applications of submillimeter waves that I'm aware of, astrophysics and high-magnetic field (EPR, ESR). Csmmpl 04:39, 4 December 2006 (UTC)
I'm afraid that the wrong part of the spectrum: instead of a band between visible and infrared, it should point between infrared and microwaves, as said in the text —Preceding unsigned comment added by 220.127.116.11 (talk) 00:12, 18 January 2009 (UTC)
- An IP has just removed the diagram with edit summary "Removed blatantly incorrect diagram", so I'll explain what the problem likely is for readers (in short it's missing an obvious infrared part). The original source image is on the right, and below it is a possibly better diagram using a log scale.
- The bottom left hand side of the top diagram shows the 100 micrometre end of the terahertz range and "points" diagonally up and to the right to the gap between the rainbow bar with the eye-icon and the 1 mm at the start of the microwave range. The diagram is very schematic. Both the visual and microwave ranges are highly exaggerated and it appears the infrared range (which should be about 1000 times as wide as the visual) is missing (or simply included in the right hand part of the rainbow bar). The two dish-like icons labeled 1 mm to 1 cm with red waves between them are intended to represent the microwave spectrum and two microwave communication dishes, and not the infrared spectrum it might at first appear. The probably better diagram underneath shows the visual part as a thin rainbow line with the infrared band to the right and the microwave band further right. The so-called terahertz should appear as a thin bar at the 10-4 to 10-3 metres wavelength range. Anyone is welcome to make a better diagram. 84user (talk) 22:47, 27 May 2009 (UTC)
"Inexpensive sources exist" sentence.
I notice that the "inexpensive sources exist in the 300-1000 GHz range" line has been tagged as needing a citation. It's going to be tough to find a direct citation for it, but it's very easy to demonstrate that it's correct. What sort of citations would be appropriate under those conditions?
Gyrotrons are cheap and high-power, widely used in t-wave experiments and for the "pain ray" that made the news a while back, but the highest frequency I've heard of is about 300 GHz. Tunnel diode oscillators are cheap and low- to medium-power, have been making the news recently, and are also widely used in experiments, but the highest frequency I've heard of is around 600 GHz. Sources that produce higher frequencies tend to involve very bulky and expensive lasers, or extremely bulky and expensive particle beams (for the free-electron laser and synchrotron sources). (A quantum cascade laser is not bulky, but _is_ quite expensive.) The only source listed that I'm not very familiar with is the backward wave oscillator, which runs at 1000 GHz and below at low power, and which I'd expect to be fairly inexpensive (like the gyrotron, it's a vacuum tube device).
I can find individual research papers using all of these devices, but finding one that says "to generate t-waves above 1000 Ghz, you need Foo types of source" will be very difficult, and none will mention the cost of the source, meaning use of such references to support a cost statement would be WP:SYN. If anyone has a more appropriate source, by all means suggest or cite it. --Christopher Thomas (talk) 19:38, 18 May 2012 (UTC)
- I'm not the one who placed the tag, but perhaps a solution would be to add to the end of the sentence: "such as gyrotrons (source), tunnel diode oscillators (source), and backward wave oscillators (source)"? Besides supplementing that claim, naming the individual devices could be of use for readers. The rest of the paragraph would still need citations, but it would help with that sentence. Khazar2 (talk) 19:58, 18 May 2012 (UTC)
Atmospheric transmission figure
A user commented that the atmospheric transmission figure needed to specify a pathlength in order for percent transmission to be meaningful. I believe the figure is okay as it is because it is the penetration through earth's atmosphere from a particular location. The pathlength, therefore, is the "thickness" of the atmosphere from that point.ronningt (talk) 15:48, 19 May 2012 (UTC)
- It's still probably worth tracking down the source it was derived from and seeing what they said about it. It may also be worth noting that it's more-or-less the absorption spectrum for _dry_ air (per the in-text question about atmospheric transmission vs water vapour in the air). --Christopher Thomas (talk) 04:08, 20 May 2012 (UTC)
Missing figures/unedited text
The second paragraph in the introduction part says:
The earth's atmosphere is a strong absorber of terahertz radiation in specific water vapor absorption bands, as seen in the two figures heading this article
The figures aren't there anymore and this needs to be fixed. Also it says ALMA is under construction in Sources > Natural! Also I don't get why Sources > Natural has a list of telescopes. Tushar Shrotriya (talk) 07:29, 23 January 2014 (UTC)
- On the second point... I think the aim was to discuss the means by which naturally occurring THz radiation is detected, since telescopes are used exclusively for that purpose. I think a solution would be to add a section on THz detector systems (bolometers, pyroelectric detectors, Golay cells etc). The telescopes that employ those detectors could then be mentioned in that section.