So my daughter never ceases to amaze me with her questions. She asked if she had a marker that only writes in UV, and she wrote on a white sheet of paper, what color (if any) would you see?
My gut reaction was to say a bluish/violet, but the more I think about it, would you see nothing since true UV is beyond our sight limits?
What does the Dope say?
(BTW this is the first thread I’ve ever started, but she tries to stump me on a regular basis. Thanks to the Dope she thinks her dad is one of the smartest humans alive! Who am I to tell her different?)
Natural light is rarely monochromatic, so a light could be most UV and still appear violet or blue because it spans a wide enough range, we just see a small part of it. Similarly with inks and dyes, it depends on whether it reflects only UV, in which case it’s invisible to humans, or else it bleeds into the visible range.
This topic has come up a few times before. The main reason we can’t see near ultraviolet is because the eye’s cornea blocks UV light. However, some early cornea replacements were transparent to UV light. The unimpressive findings were that UV just looks whitish-blue or whitish-violet. When my father had cataract surgery, I asked him about this, but he said the doctors told him that modern cornea replacements are now designed to block UV rays.
We (daughter and I ) read about this. She said she really wants to be able to see like the birds and the bugs, but doesn’t want to resort to surgery…yet.
Note that most stuff that’s visible under UV is fluorescent (like the linked markers)–which is to say that the dye takes in UV light and converts it to a frequency that we can see. So that doesn’t really count for these purposes.
Look at these pictures of flowers. You would never know it from the visible light image, but there is hidden detail in UV.
To be clear, the colors they chose for the UV image are arbitrary–it’s impossible to say what this would “really look like” if you could see UV. The point is that the dyes that the flower used look identical in the (human) visible spectrum, but reflect differently in UV. Likewise, one could in principle have a marker that was invisible in visible light but reflected UV differently.
Typically, these things actually fluoresce in the visible band when excited by UV illumination - that’s why we can see them. A UV-coloured dye that only reflects UV light would appear black under normal visible illumination, whereas under invisible UV illumination, it would appear black, because we would not be able to see the invisible UV light being reflected off it.
You need to start a conversation about what “a marker that writes in UV” means.
A “red” marker dispenses red ink, which blocks green and blue light, but transmits red light. Which is why the red light can pass through the ink, get reflected by the underlying paper, reflect back and make it to your eyes. All other colors get blocked by the ink. So the mark looks red.
If it’s a “UV marker” in the same sense, then it would block all visible light colors and transmit UV. We can’t see the UV, so it would just look black.
As Dr. Strangelove said, this is a fluorescent marker. It absorbs UV and converts it to visible light. It’s not really a “UV marker” in the same sense of a “red marker.”
UV markers contain fluorescent dyes: They absorb light in the UV range, therefore not visible to the naked eye. However, when excited by UV light of the correct wavelength, they emit light in the visible range of the spectrum.
We probably need to differentiate between transmission and reflection too (not that I disagree with anything you wrote). A ‘dye’ type red marker works as you describe above - a ‘paint’ type red marker appears red because it contains pigments that reflect red in their own right, and absorb everything else.
There’s probably also a third case for dyes that transmit some wavelengths and reflect others (in theory, such a thing, painted on glass, would appear red from the lit side and greenish blue when held up to the light and looked through.
also, there’s a (small) chance such a marker wouldn’t be that discernable under a black light; a lot of paper has UV brighteners in it which make it look “more white” and also make it fluorescent when a UV light shines on it.
Her question is like the premise of the Ambrose Bierce story “The Damned Thing” about a monster that was infrared in color, and which therefore was invisible to human eyes.
The problem is that you can have something that reflects in the ultraviolet or the infrared, it still has to do something with visible light, as well.
If your daughter’s pen writes in ink that doesn’t affect visible light, but does reflect UV, then it will be invisible to human eyes, but you could see it with an ultraviolet camera. The same goes for infrared (Bierce’s monster wouldn’t be invisible, though – a monster still has to do SOMETHING to visible light, unlike writing with your daughter’s pen) There are inks, in fact, that do exactly this – they’re used as anti-counterfeiting agents.