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Upon looking at some radiation levels higher than Microwaves, I have come across "Terahertz radiation". According to the article, there are numerous ways to generate such radiation, and that brings questions to me.

My confusion lays in how it is different (if it is!) from a heating fan emitting infrared radiation (simple), or an LED emitting visible green light, or a flashlight emitting various spectral frequencies. Are those examples emitting Terahertz radiation? I would hypothesize that is, but can see no reference to light being called "Terahertz radiation" even though it is radiating.

Although I am unsure if the technologies listed in the article can go above infrared in to visible light, but assuming so, would it emit "light" (in the respective frequencies)? How would this be different than an ordinary visible light emitter? At that point is it no different from an "ideal" colour changing LED?

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As the Wikipedia article you linked to says,

Terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. [...] The term typically applies to electromagnetic radiation with frequencies between high-frequency edge of the microwave band, 300 gigahertz (3×1011 Hz), and the long-wavelength edge of far-infrared light, 3000 GHz (3×1012 Hz or 3 THz).

So this radiation is simply a particular type of electromagnetic radiation, identified by the fact that it has a frequency within a particular range. Other than the frequency (and consequently the wavelength and photon energy), it's no different from any other electromagnetic radiation.

The EM radiation that makes up visible light has frequencies about a thousand times higher than this range. That's why visible light is not called terahertz radiation. And (most of) the infrared radiation emitted by objects at the temperatures typical on Earth has a frequency around 10 times higher than the terahertz range.

If some device that emits terahertz radiation could have the frequency of that radiation adjusted upward by a factor of a thousand or so, it would be emitting visible light. The spectrum (and color) of the light emitted would depend on the device.

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Ah I see, it just isn't classified past that. Visible light is in the THz range though (only ~100x the upper limit of that classification, >=400THz) and is arguable "terahertz radiation", just as some infrared is arguably micrometer/THz radiation. Would anything at the frequency of visible light be a photon, because of the energy? – Xander Apr 28 '12 at 8:07
But in David's answer, the top end of the "THz range" is 3THz. Anything higher isn't considered THz radiation. The fact that you can express it in THz isn't relevant. The photon concept applies to all electrmagnetic waves regardless of the frequency. – twistor59 Apr 28 '12 at 8:11
@twistor59, calling it THz radiation (and not infrared/submm) just leaves things out. On Wikipedia there looks like a better explanation to it: "However, typically, the term "terahertz" is used more often in marketing in relation to generation and detection with pulsed lasers, as in terahertz time domain spectroscopy, while the term "submillimeter" is used for generation and detection with microwave technology[...]". Sub-mm is probably a better name for 0.1-1mm, because <0.1mm is too far below to consider "sub-mm", more like upper nm radiation, yet still in the THz range. – Xander Apr 28 '12 at 8:14
Yes, this nomenclature is certainly a bit arbitrary! – twistor59 Apr 28 '12 at 8:21
Glad someone could agree on me that it is at fault and not my understanding. I accept this answer, and find it useful to my sanity. :) – Xander Apr 28 '12 at 8:29

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