What would seeing UV light tell you ?

[madd0ct0r]

I was thinking the opposite. Ink that takes visible light and reflects uv that you can see. Easy to track a car from above when it has a glowing x on it.

[jwl]

I was thinking the opposite. Ink that takes visible light and reflects uv that you can see. Easy to track a car from above when it has a glowing x on it.

I don't think it's easy to do that, because of how florescence works. There's a reason you need blue LEDs to make white ones.

[Broomstick]

An object can reflect UV without being florescent. This effect occurs all the time in flowers and the feathers of birds, we just can't see it without special cameras.

Birds and insects, on the other hand, can. To them it's just another color.

[jwl]

An object can reflect UV without being florescent. This effect occurs all the time in flowers and the feathers of birds, we just can't see it without special cameras.

Birds and insects, on the other hand, can. To them it's just another color.

Yes, but turning visible light into UV light isn't something that happens all the time. To turn UV into visible the UV excites an electron to a higher energy level, and as it falls it produces visible light. But try the same trick the other way around and the visible light doesn't have enough energy to excite the electron to the higher energy level.

[madd0ct0r]

https://en.m.wikipedia.org/wiki/Stimulated_emission

I'm not a physicist, but the article does seem to imply that energy levels of the outward photons can be shifted up or down

[jwl]

https://en.m.wikipedia.org/wiki/Stimulated_emission

I'm not a physicist, but the article does seem to imply that energy levels of the outward photons can be shifted up or down

Stimulated emission is the thing you get in lasers, it isn't easy to do. It works by the photons causing a drop in the energy level of electrons that are already excited, which requires something to have excited them already, usually an applied voltage or high temperatures. Note that if stimulated emission didn't happen, the electrons would fall down anyway via spontaneous emission, which is what happens in neon tubes and CFLs.

In fluorescence, the electrons are not already excited, and get excited by UV. But you can't get excited by a low-energy photon and give off high-energy photons, because then energy wouldn't be conserved. So you can get UV->visible but not visible->UV. That's why you need blue LEDs to make white ones: the fluorescent coating on the blub can turn blue/UV light to lower frequencies, but not the other way around.

[Terralthra]

https://en.m.wikipedia.org/wiki/Stimulated_emission

I'm not a physicist, but the article does seem to imply that energy levels of the outward photons can be shifted up or down

Stimulated emission is the thing you get in lasers, it isn't easy to do. It works by the photons causing a drop in the energy level of electrons that are already excited, which requires something to have excited them already, usually an applied voltage or high temperatures. Note that if stimulated emission didn't happen, the electrons would fall down anyway via spontaneous emission, which is what happens in neon tubes and CFLs.

In fluorescence, the electrons are not already excited, and get excited by UV. But you can't get excited by a low-energy photon and give off high-energy photons, because then energy wouldn't be conserved. So you can get UV->visible but not visible->UV. That's why you need blue LEDs to make white ones: the fluorescent coating on the blub can turn blue/UV light to lower frequencies, but not the other way around.

There are a few situations (inverse Compton scattering) where it works in reverse and emits higher energy photons than came in, but they usually involve relativistic phenomena.

[jwl]

https://en.m.wikipedia.org/wiki/Stimulated_emission

I'm not a physicist, but the article does seem to imply that energy levels of the outward photons can be shifted up or down

Stimulated emission is the thing you get in lasers, it isn't easy to do. It works by the photons causing a drop in the energy level of electrons that are already excited, which requires something to have excited them already, usually an applied voltage or high temperatures. Note that if stimulated emission didn't happen, the electrons would fall down anyway via spontaneous emission, which is what happens in neon tubes and CFLs.

In fluorescence, the electrons are not already excited, and get excited by UV. But you can't get excited by a low-energy photon and give off high-energy photons, because then energy wouldn't be conserved. So you can get UV->visible but not visible->UV. That's why you need blue LEDs to make white ones: the fluorescent coating on the blub can turn blue/UV light to lower frequencies, but not the other way around.

There are a few situations (inverse Compton scattering) where it works in reverse and emits higher energy photons than came in, but they usually involve relativistic phenomena.

Well yeah, but you aren't going to put anything capable of inverse Compton scattering on the roof of your car (and besides, it wouldn't just produce UV light via inverse Compton scattering, it would also be glowing really brightly in the visible spectrum).

[madd0ct0r]

Humph. Conceded.

[Sea Skimmer]

I think the basic reality would be a UV marking won't be brighter then a visible light marking, but it would be far more covert for day to day purposes. And that's pretty significant. UV certainly has a lot of military interest, long has, for overhead images. Its less relevant ground-ground viewing.

Of course on the other hand viewing in UV is kind of light viewing in black and white. Contrast shows out very strongly, but differences of visual color can be entirely lost. So maybe you can tell that car has a UV X on the top, but you might not know what color the car actually is if you were driving behind it. All the more so if we are speaking of a UV eye function which simply feeds a visual wavelength image to the human brain, rather then the more profound change require to rework the human visual processing centers to actually be optimized for UV information. Id merely speculate the later would work better, the practical distinction might not be much.