UV radiation isn't visible to the human eye, so how come we can see it as a purple/violet light from a UV lamp? Is it just because the lamps aren't perfect and end up emitting some light at a higher frequency? Or do they add some purple light intentionally? Or is there some more complex mechanism going on?
[Physics] Why do UV lamps look purple
electromagnetic-radiationeveryday-life
Related Solutions
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.
You are thinking in terms of atoms and molecules and you are mainly talking of solid state matter .
Solid state is another quantum mechanical phase, it has lattice structure with much smaller energies than atomic and molecular transition structures. Lattices have vibrational levels which are mainly responsible for the black body radiation solids emit, infrared is also photons.
A rule of thumb with radiation impinging on solids is that if the wavelength is smaller than the lattice dimensions the photons can penetrate easily the lattice, interacting only with direct scatters hence the higher penetration of X rays and gamma rays. Here is an article that discusses the penetration of radiation, X rays and higher.
For glass and optical frequencies there is a good answer here in this site., essentially the structure of the transparent materials is such that the photons pass through without loosing energy in the visible.
For infrared where the wavelengths are large in comparison with lattices or distances between molecules in liquids, the photon can give up its energy in collective excitations at the surface gradually heating up the material.
For ultraviolet, glass, depending on the type, has some absorptive bands, the photon energy transferred at the surface to collective modes or breaking molecular bonds and transformed to heat ( infrared) further in.
So your
Once you reach a critical frequency, however, the photons will begin to be absorbed because they have enough energy to excite the electrons (which is why glass is opaque in ultra-violet).
has small probability to happen until x-ray energies are reached which are the energies of bound electrons, and the link above gives the dependence in a simplified manner.
Best Answer
There are several kinds of "UV Light." Only kind I know of that "looks purple" is a so-called black light.
Black lights emit UV that is very close to the top-end of the visible spectrum. The designers try to minimize the visible radiation so that it won't wash out the light emitted by fluorescent substances in the field, but it's hard to filter all of the visible out.
The visible tail looks "purple" because the "red" receptors in your eye have some sensitivity at the shortest visible wavelengths. The visible leakage from a black light stimulates both "red" and "blue" receptors in your eye, and you perceive purple.
Other types of UV lamp (e.g., germicidal lamps) generally put out a lot of visible light that they don't bother to filter out because (A) it does not interfere with the application (e.g., visible light won't stop the UV from killing the germs) and (B) for safety reasons (i.e., so you know when you're looking at the light, which you shouldn't because it's dangerous.)