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So in my world interdimensional travel produces UV light. However, there are obviously other things which produce UV light/reflect it and look different. Being a non-physics person, can someone explain what looks particularly different when viewed with UV light?

Some ideas I have are that the night sky, flowers and electricity would look particularly different. By the way, I'm looking for particularly big differences, not small ones such as "this looks a little bit more purple."

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    $\begingroup$ Relevant: youtube.com/watch?v=V9K6gjR07Po $\endgroup$ – plasticinsect Sep 4 '18 at 20:11
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    $\begingroup$ UV cameras exist you might want to look at some of the pictures they take. for instance makeup is really obvious. $\endgroup$ – John Sep 4 '18 at 20:57
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    $\begingroup$ How does your character's UV vision work? Does she see it as a fourth color? Or does it replace one of the other colors that normal people see? Or, does she only see UV, making the world black and white (no-UV and lots of UV)? $\endgroup$ – DqwertyC Sep 5 '18 at 0:31
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    $\begingroup$ Ask a butterfly. They see UV as well as RGB. $\endgroup$ – RonJohn Sep 5 '18 at 4:56
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    $\begingroup$ People with partial or total aphakia (the lack of a lens in the eye) can sometimes see into UV. Their brain apparently usually processes it as "bluish-white" or "purplish-white". Unfortunately, they're also highly farsighted, unable to change their focus point, and photosensitive, so it's not exactly a superpower—but it does mean they can see UV lamps, and there are some reports of seeing glows from UV sources in the dark, but in normal light it just means everything look brighter and blurrier. (Look up Bill Stark from St. Louis University, a vision researcher who's aphakic in his left eye.) $\endgroup$ – abarnert Sep 5 '18 at 7:00
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Assuming that the ability to see into the UV spectrum is an additional, fourth elementary color, and that red, green, and blue vision remain unaffected, keeping this person's ability to see all of the colors that they can see now, in addition to seeing UV...

It would be a whole new color. There really isn't much room in human understanding to comprehend new colors. The best that UV sensitive cameras can do is translate UV into a range of colors that we're familiar with.

One of the reasons why humans don't have UV vision is that it's a wavelength of light that gets scattered quite a bit by our atmosphere. So, anyone who has UV vision would see a uniform UV colored haze during the day when outside, and just as hazy, cloudy weather reduces shadows, making everything look somewhat more flat and drab, this is what the UV color would look like all the time: rather uniform with few shadows.

The biggest thing we'd notice is that phosphorescents -- things that glow to humans under black lights, will be noticeably darker.

This is because the phosphorescent surface is absorbing the UV light and re-emitting at a different color. There will always be a loss of energy in this conversion.

There are some plants that generate stripes that can only be seen in UV ranges.

Clothes washed in detergent that is meant to make clothes brighter will look much more drab and dirty. These detergents use a phosphorescent residue.

Some glass will be completely opaque to the UV color... just like different colored glass is opaque to other colors... For instance, red glass blocks green light. (Digital cameras use glass that is opaque to IR and UV on purpose, to pre-filter to just the band that is visible to humans.)

There would be little that's "new" that you would see. You'd still see a rock on the ground; it would just have a new color depending on its surface properties. Some few things that look identical to normal human vision might have differences to your expanded-color-vision person. For instance, they'll be able to tell at a glance when someone is wearing sunscreen, and can easily tell just how effective that sunscreen is.

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    $\begingroup$ I'd like to add a link about those plants: bristol.ac.uk/botanic-garden/news/2013/49.html There'd probably be a lot more appreciation for flowers that specialize in bees and other UV-sensitive insects for pollination. Especially some of the simple, yellow flowers would just become beautiful. $\endgroup$ – Jacco van Dorp Sep 5 '18 at 8:59
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    $\begingroup$ if i recall correctly, this would mean that rainbows are also significantly extended in size $\endgroup$ – Erin B Sep 5 '18 at 14:23
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    $\begingroup$ While phosphorescents might be darker in the UV band they’d be brighter in other colours. Given how the human brain deals with colour you might expect phosphorescent materials to really ‘pop’ in their non-uv colour because of the relative increase in brightness $\endgroup$ – Joe Bloggs Sep 5 '18 at 16:13
  • $\begingroup$ The person would also need their lenses in their eyes to be made of something else as the human lens blocks most UV light. And in fact a human without a lense at all would be able to detect some amount of the UV spectrum in real life (of course it would be blurry vision without your lense). $\endgroup$ – Shufflepants Sep 5 '18 at 17:54
  • $\begingroup$ @JoeBloggs, yes, just as phosphorescent materials 'pop' in the other colors for people who don't see UV, already. The difference is, if you have a blacklight, everything is already bathed in light that you can see, just as if you were standing in an area bathed in green light. If there were things that, when under green light glowed red instead, and you only had green lights around, the red-glowing object would still be dimmer than a white wall. It would look very oddly red in that context, and "UV-glow" objects would look oddly colored but still dimmer than a white wall. $\endgroup$ – Ghedipunk Sep 5 '18 at 18:46
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Your specific question is, "what could a UV-sighted person see that the rest of us can't?" While there are many things that such a person would see differently than we (things that we would also see, but would perceive differently). Therefore, I reject fluorescing rocks because we limited beings can see the rocks just fine.

Dried or wet, there are some invisible things a UV sensitive person could see (basic list from here:

  • Saliva & urine
  • Bleach and some liquid detergents
  • Liquids containing quinine including club soda
  • Lemon juice (though this isn't quite invisible to our regular eyes)

Blood is an interesting topic. By itself we can see it wet or dry. But if cleaned simply with soap and water, it's no longer visible to us, but would be visible to someone capable of seeing in the UV spectrum.

I'm not a chemical engineer, so I can't tell you what compounds are involved (other than quinine, which I found quickly online), but that list of compounds is what would be of greatest interest to you.

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  • $\begingroup$ many substances have different colors in UV that we can't detect many birds and flowers have patterns we can't see for instance. From UV cameras we know many materials we think of as transparent are opaque or close to it in UV, one of my favorite images is a picture of a woman who looks like a mime because her concealers looks like black face paint in UV. $\endgroup$ – John Sep 5 '18 at 3:26
  • $\begingroup$ @John, but you can see the bird and you can see the woman. IMO, the OP's question is best answered with things we three-coners can't see at all. Perhaps we need the OP to clarify his/her intent. $\endgroup$ – JBH Sep 5 '18 at 4:49
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    $\begingroup$ there is nothing we can't see at all you can see with UV, They just look different, drastically different in some cases, some newer windows would not be transparent for instance. $\endgroup$ – John Sep 5 '18 at 14:13
  • $\begingroup$ Club soda doesn't contain quinine. Perhaps you're thinking of Indian tonic water? $\endgroup$ – Jules Sep 6 '18 at 16:08
  • $\begingroup$ @Jules, Possibly :-), as mentioned in my post, I found those on a list onine and am not, myself, a chemical engineer. $\endgroup$ – JBH Sep 6 '18 at 16:44
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This resource has a lot of options: https://www.thoughtco.com/what-glows-under-a-black-light-607615

Some of the most interesting, in my mind, were:

  1. Plants (or the chlorophyll in them) glows red
  2. Scorpions glow bright blue-green
  3. Body fluids also glow

People also glow under UV light, which might be an interesting skill for your protagonist to have.

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    $\begingroup$ what something looks like under a black light is not the same as what it would look like in UV. $\endgroup$ – John Sep 4 '18 at 20:56
  • $\begingroup$ The article says "A black light gives off highly energetic ultraviolet light". $\endgroup$ – Caleb Syring Sep 5 '18 at 3:20
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    $\begingroup$ which our eyes cannot see, we only see the few substances that absorb UV and emit something with a lower wavelength. $\endgroup$ – John Sep 5 '18 at 3:22
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    $\begingroup$ Exactly what John says. It's like shining a red light at a wall and seeing green instead. If you can see into UV, then the objects that shine brightly under a blacklight will be very dark in the "UV color" that a person sees. Something that glows in the human optical range only glows because energy is stolen from the uv range. $\endgroup$ – Ghedipunk Sep 5 '18 at 3:44
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The link by Caleb Syring is excellent.

The useful bits for Sci-Fi scenario could be:

  • Can tell humans from humanoid robots (b/c plastic skin glows, and human skin absorbs UV). Same for artificial limbs. I know I can see all the tooth fillings and crowns in my mouth under UV light.

  • Can see traces of blood on vampire's lips, even if they are wearing red lipstick. Or can see traces of bloody massacre even after it was cleaned up.

  • Can see mold before it becomes visible to humans. Including the bioengineered mind-altering mold.

  • Can see traces of slime left by monsters or aliens.

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    $\begingroup$ Actually, a common misconception about UV light is that it can be used to see blood, even after it's cleaned up. In reality, human blood does not fluoresce under UV light, although there are other ways to make it shine brightly. $\endgroup$ – Nzall Sep 5 '18 at 13:00
  • $\begingroup$ What does leave stains that show up under UV light, however, is semen… $\endgroup$ – Draconis Sep 5 '18 at 16:32
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Almost nothing, under every day circumstances

Our atmosphere blocks out almost all UV light - this is why the other answers are talking about shining UV light on things.

Q: If you get a UV camera and just point it out your window, what does it pick up? A: Practically nothing - there will be more input from electrical noise in the camera affecting the sensor than there will be UV input, unless you point it directly at the sun.

Your character will be able to see UV sources directly, without needing things to flouresce to detect its presence. electrical arcs, blacklights, rocket motors etc would be more visible to them.

Wikipedia notes that some people can see UV light

Ultraviolet rays are invisible to most humans. The lens of the human eye blocks most radiation in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. Humans lack color receptor adaptations for ultraviolet rays. Nevertheless, the photoreceptors of the retina are sensitive to near-UV, and people lacking a lens (a condition known as aphakia) perceive near-UV as whitish-blue or whitish-violet. Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm.

For Near UV light vison, this will be like having near IR vision - like a cheap camera such as you will find in your phone or as a webcam has - it'll pick up on e.g. a remote control for a TV (which uses near IR) as a glow, but it can't see beyond that - in the IR space that would be thermal vision.

There still isn't much UV light that makes it to the surface of the earth. These people are able to see the light from UV sources primarilly.

For UV, being able to see beyond near UV light, you're passing out of the range covered by blacklights to the shorter wavelengths, that are absorbed by the Ozone layer.

Ozone absorbs more than 99 percent of UV-C rays -- the most dangerous portion of the spectrum. Ozone absorbs about 90 percent of the UV-B rays

sciencing.com

you're not going to see much at all in this region, even if you block out all of the visible light.

The main reason we have evolved to see the visible spectrum is because that range is mostly transparent to our atmosphere - the light in that range reaches us. Having a larger visual range is not beneficial, as the intensity drops off rapidly: Atmospheric transmission, from: http://gsp.humboldt.edu/olm_2015/Courses/GSP_216_Online/lesson2-1/atmosphere.html

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    $\begingroup$ Glass blocks UV but atmosphere don't. Otherwise you wouldn't need UV filters. $\endgroup$ – Ister Sep 5 '18 at 13:25
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    $\begingroup$ @Ister have you heard of the Ozone layer? Ozone absorbs more than 99 percent of UV-C rays sciencing.com/percent-uv-ozone-absorb-20509.html the absorption of the less ionising rays (closer to visible light) is less, but still significant. White cotton readily flouresces under a black-light, but not in the sun - there isn't enough of it. We need UV filters because UV-b and UV-c is harmful - even small amounts of it are absorbed and it causes chemical reactions to occur, such as directly to our DNA. It doesn't require a high level to do damage $\endgroup$ – Baldrickk Sep 5 '18 at 13:47
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There are already some great answers, I'd like to point out a few more things that they would be able to see. Mostly, I'm thinking of manufacture pigments that are designed to trick people who see visual light, but wouldn't fool someone who can see ultraviolet.

  • False teeth, which are designed to look identical to natural teeth are often different colors in UV.
  • Makeup, it might match the skin very well in visibly light, but it's extremely obvious in UV
  • You can tell who is wearing sunscreen
  • You might be able tell what has been repainted when other people can't
  • Some freckles are only in UV, you'll disagree with other people about who is covered in freckles
  • You might be better at identifying natural sunlight

Maybe it's not amazingly useful, but you could leave hidden messages for your UV seeing friends. You wouldn't need special UV paint, just put zinc white over titanium dioxide white.

I also think it's worth pointing out that you'd loose most of these abilities indoors, especially under incandescent and LED lights. Florescent light bulbs have more UV, but if there is no UV light source in the room, everything would look the same to you as to everyone else.

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  • $\begingroup$ Expanding on your last paragraph: you'd probably find that most indoor spaces were much darker than outdoor ones, to a much higher extent than anyone who couldn't see UV, because we calibrate how much lighting to use based on how much visible light it produces, but a UV-sensitive would naturally expect to be able to see more light. Alternatively, she might find being outdoors during the day unbearably bright. $\endgroup$ – Jules Sep 6 '18 at 16:14
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In addition to the above items, this person may also be able to "see" radioactivity, after a fashion. Alpha particles, Beta particles, X-rays and gamma rays all have modes by which they will produce UV upon interacting with air or matter.

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Visiting an art museum might have some interesting results...

Many old masterpieces look quite different when viewed with X-Ray, UV, Infrared, or other special cameras.

https://www.vice.com/en_us/article/5995pb/7-hidden-art-secrets-that-were-uncovered-with-technology

I would also note that UV light often harms a normal person's eyes. So in addition to being able to see UV light their eyes would probably benefit from a resistance to its harmful effects. Otherwise they wouldn't be able to use that ability very much.

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  • $\begingroup$ Direct exposure to any strong light is harmful to your eyes. The trick is you don't see the strong light when it's in UV range so you're not aware of the danger. You (like all other people and animals) a're constantly exposed to the UV light. It's just not that strong. But if you look directly into the Sun direction you might damage your sight, and believe me or not it is not just a matter of UV light, but regular visible (to us) light as well. $\endgroup$ – Ister Sep 5 '18 at 9:33
  • $\begingroup$ @Ister - no, UV does more damage per photon than other light because (1) it is shorter wavelength and therefore carries more energy in each photon, (2) it is absorbed strongly by DNA, and (3) because you don't perceive it, your eyes don't adjust aperture to reduce the amount that gets in like they do for visible light. The same is also true for a lesser extent for blue light (which doesn't trigger the reaction as well as other colours). $\endgroup$ – Jules Sep 6 '18 at 16:22

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