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Thread: Black is best UV reflector?

  1. #1 Black is best UV reflector? 
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    Could somebody explain why black color is good
    ultraviolet reflector when it does absorb IF and visible
    light well?Where could I find a chart that shows reflectivity of different colors to UV?


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  3. #2 Re: Black is best UV reflector? 
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    Quote Originally Posted by Stanley514
    Could somebody explain why black color is good
    ultraviolet reflector when it does absorb IF and visible
    light well?Where could I find a chart that shows reflectivity of different colors to UV?
    If it is truly black it absorbs everything and reflects nothing. That is in fact the definition of black.

    What you are apparently talking about is some material that absorbs visible light but reflects UV. That would be a property of the specific material, not just the color.


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    Some scientists believe that dark or black skin evolved
    to protect against high UV exposer.It seems a compromise to me because dark-skinned people will
    more suffer from Infrared and visible light which will not
    be reflected.Could there be a material which reflects
    everything?
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  5. #4  
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    The melanin in dark skin intercepts UV by safely absorbing it.
    A pong by any other name is still a pong. -williampinn
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    The melanin in dark skin intercepts UV by safely absorbing it.
    In this case it makes no sense to me at all.
    It should be much better to have in sunny climate something that reflects all parts of spectrum,not absorbs it!Possibly, some kind of white material.
    Because energy can't go somewhere it will convert in
    heat inside of skin.For example, white material reflects
    95% of rays and black absorbs 95%.It means black-skinned person will suffer 20 times more from heat than white.I just wander how they didn't perish in their
    hot countries!(in the times when there was no white cloth,naturally)
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  7. #6  
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    Well, skin is not a 2-dimensional plane like a mirror. Under a microscope skin's a rough porous landscape of dead tissue... flaking translucent layers upon layers of junk that simply can't mutate because it's dead. So, randomly bouncing UV around in there is not helping. Better to absorb UV before it penetrates to living cells.
    A pong by any other name is still a pong. -williampinn
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    Better to absorb UV before it penetrates to living cells.
    This is only if naked person will not die sooner because of heat under bright sunlight than because of skin cancer.And there would be no way to get UV in to living cell if it would be surrounded by white reflectors tightly enough.White cloth is also not 2-d plain mirror but reflects light quite well,better then black cloth.
    It's difficult to imagine for me how those almost naked
    people hunted hypos under burning African sun for tenths of thousands of years and more vulnerable of
    them survived.
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  9. #8  
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    Now I too am confused. We need a physics person.
    A pong by any other name is still a pong. -williampinn
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  10. #9  
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    Quote Originally Posted by Pong
    Now I too am confused. We need a physics person.
    No, you need a physician. The role of skin pigment has more to do with things other than heat transfer, like the manufacture of vitamin D in sun light, and resistance to sunburn.
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    And also: UV light is known to be dangerous because it
    cause ionization of matter.If it has sufficient energy to ionize atom,how could it be harmlessly absorbed?Will it not ionize all the same?And UV light still need to penetrate trillions of molecular layers to get
    to DNA.So if all those molecules will be white and not transparent.So there is no chance for UV to get so deeply if only living cell is not completely transparent.
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  12. #11  
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    Dr. Rocket please re-read the exchange between Stanley514 and myself. Then respond as a physicist and as a person.
    A pong by any other name is still a pong. -williampinn
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  13. #12  
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    I doubt the melanin minds being ionized nearly as much as your DNA does.
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  14. #13  
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    Here's one good reason to have pigmentation in sunny areas:

    Pyrimidine Dimers

    As mentioned before, melanin absorbs UV light and gives off the energy as heat (which does not harm DNA).

    edit: it's also important to note that evolution is not perfect, it really is just a trial and error process. So far melanin is the best solution. In order to select for something "better" it would need to be produced through mutations first. Of course, if we had been designed by an intelligent creator then I could see why such imperfections don't make any sense...
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    Pro tip: Only use sunblocks with metal oxide nanoparticles. Don't use organic sunblocks. There is substantial research showing that they actually increase the likelihood of dangerous skin cancers.
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  16. #15  
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    Quote Originally Posted by Scifor Refugee
    Pro tip: Only use sunblocks with metal oxide nanoparticles. Don't use organic sunblocks. There is substantial research showing that they actually increase the likelihood of dangerous skin cancers.
    How do you know that metal oxide nanoparticles are safer?
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    Quote Originally Posted by gc
    How do you know that metal oxide nanoparticles are safer?
    Various studies have shown that the metal oxide nanoparticles don't get absorbed into the skin like organic sunblocks do; the nanoparticles simply sit on the surface of the skin. The organics soak deep into the skin, and then generate dangerous oxidizing and reducing species upon irradiation.
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  18. #17  
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    Quote Originally Posted by Scifor Refugee
    Quote Originally Posted by gc
    How do you know that metal oxide nanoparticles are safer?
    Various studies have shown that the metal oxide nanoparticles don't get absorbed into the skin like organic sunblocks do; the nanoparticles simply sit on the surface of the skin. The organics soak deep into the skin, and then generate dangerous oxidizing and reducing species upon irradiation.
    What about silicone? I have seen some high end tanning lotions that include silicone.

    Now I am one to believe there is more benefit from the sun than harm. The changes of cholesterol to Vitamin D is not the only metabolic change. Don't get me wrong, this needs to be done in moderation. I have read studies that point out that even though there is a higher risk of skin cancer in those who tan, there are far lower cases of other bodily cancers. Can anyone confirm or deny this?

    I am a sun worshiper. I am careful not to let myself burn. I don't use any sun blocks either. I also like to soak in a hot bath with sea salt after-wards to help extract some toxins produced. I get that seasonal disorder thing, and tanning once a week really helps my mental state, otherwise I get rather bipolar. I actually haven't tanned this winter, and have been rather unforgiving with some people.

    I'm going to go tan today!
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    Quote Originally Posted by Wild Cobra
    What about silicone? I have seen some high end tanning lotions that include silicone.
    I've never heard about Si-based sun blocks, so I don't really know. I suspect that the silicone might have been included to change the texture or help it spread better or something.
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    It would be interesting to know if there is some animals
    which skin or fir works exactly like this: efficiently reflect UV light instead absorbing it.It seems that some animals in Africa such as lions prefer to have light fir instead being "naked".There is lot of white birds,I guess,in hot countries.So there is many mechanisms for sun protection which nature developed.
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  21. #20  
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    Quote Originally Posted by Stanley514
    It would be interesting to know if there is some animals
    which skin or fir works exactly like this: efficiently reflect UV light instead absorbing it.It seems that some animals in Africa such as lions prefer to have light fir instead being "naked".There is lot of white birds,I guess,in hot countries.So there is many mechanisms for sun protection which nature developed.
    If you think animals evolved with reflectance and absorptance of their skin as the primary driver, then explain the zebra. When you are done with that explain why all animals in a fixed enviroment are not the same color.

    Do you think that maybe lions are the color of dry grass because they hunt in dry grass ?
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  22. #21  
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    Anyway human skin does build protective melanin in response to UV. We tan.

    The question Stanley514 raised, which at this point I think a physicist could answer, is: wouldn't reflective skin be better?

    I think the problem with reflection is the light would bounce around inside the (porous, dead) layers so more would penetrate to living cells.
    A pong by any other name is still a pong. -williampinn
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  23. #22  
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    Quote Originally Posted by Pong
    Anyway human skin does build protective melanin in response to UV. We tan.

    The question Stanley514 raised, which at this point I think a physicist could answer, is: wouldn't reflective skin be better?
    Better for what ?

    When we tan we make vitamin D. That is important.

    "Better" or "best" require that you have some objective function to optimize.
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    Okay... given that skin builds rough translucent dead layers, which may either contain embedded specks of reflective or absorptive material (as design option), and given the objective of limiting the UV radiation penetrating through to living cells... which is "better" - reflective specks or absorptive specks?
    A pong by any other name is still a pong. -williampinn
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  25. #24 Re: Black is best UV reflector? 
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    Quote Originally Posted by Stanley514
    Where could I find a chart that shows reflectivity of different colors to UV?
    Birds, Bees, flowers.
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  26. #25  
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    Also, Polar bears are white, not because their fur is white, but because it reflects so much of the light hitting it, the same as snow. This is probably for camouflage reasons, as they tend to overheat and have to roll around in the snow to cool down.
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  27. #26  
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    Quote Originally Posted by Wild Cobra
    Now I am one to believe there is more benefit from the sun than harm. The changes of cholesterol to Vitamin D is not the only metabolic change. Don't get me wrong, this needs to be done in moderation. I have read studies that point out that even though there is a higher risk of skin cancer in those who tan, there are far lower cases of other bodily cancers. Can anyone confirm or deny this?
    If you're looking for vitamin D you could always take a supplement, or eat lots of fish such as salmon (good for you in other ways too). As for SAD, I know that light is often used as a therapy but I don't know exactly how that works, but surely there are more healthy ways than a tanning bed?
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  28. #27  
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    Quote Originally Posted by gc
    Quote Originally Posted by Wild Cobra
    Now I am one to believe there is more benefit from the sun than harm. The changes of cholesterol to Vitamin D is not the only metabolic change. Don't get me wrong, this needs to be done in moderation. I have read studies that point out that even though there is a higher risk of skin cancer in those who tan, there are far lower cases of other bodily cancers. Can anyone confirm or deny this?
    If you're looking for vitamin D you could always take a supplement, or eat lots of fish such as salmon (good for you in other ways too). As for SAD, I know that light is often used as a therapy but I don't know exactly how that works, but surely there are more healthy ways than a tanning bed?
    I didn't mean to turn this into a tanning thread, but supplements simply are not as good as natural. I do take supplements from tome to time, but refuse to rely on them. I simply take them for what I might accidentally miss out on. The supplements I take are amino acids and enzymes rather than vitamins, though I do pop vitamins maybe once every two weeks.

    As I said before, there is more going on with tanning than just vitamin D. Even if you could prove it as just a placebo effect. I enjoy the sun.
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  29. #28  
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    Quote Originally Posted by Wild Cobra
    As I said before, there is more going on with tanning than just vitamin D. Even if you could prove it as just a placebo effect. I enjoy the sun.
    I didn't mean to imply that you shouldn't enjoy the sun. Who doesn't enjoy a nice sunny day? I just thought I should point out that if the purpose of getting extra UV radiation (through the sun or tanning beds) is to get more vitamin D, there might be easier ways.
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  30. #29  
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    Quote Originally Posted by Pong
    Okay... given that skin builds rough translucent dead layers, which may either contain embedded specks of reflective or absorptive material (as design option), and given the objective of limiting the UV radiation penetrating through to living cells... which is "better" - reflective specks or absorptive specks?
    I would make an educated guess that absorptive specks would be better.

    That is because once light hits an absorptive speck the game is over.

    When light hits a reflective speck it goes somewhere else. If it goes away from the body then the game is over. If it reflects to some other speck it could go out or penetrate further. So now you have a random walk of the light ray going on and some of the reflected light will penetrate.

    So, if you have absorptive specks uniformly distributed that intercepted any incoming light ray, there would be zero penetration, but if the specks uniformly distributed and randomly oriented, some would penetrate. The degree of penetratin with the reflective specks would depend on the depth of the layer.

    A mirror skin would be a different thing. In that case all of the light would be reflected away and you would have no penetration, and with no absorption no heating either.
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  31. #30  
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    Quote Originally Posted by Wild Cobra
    I enjoy the sun.
    Did you know one of the great traditional joys of Christmastime in Canada is when the mandarin oranges arrive? We devour them like drugs. It's the shot of vitamin C. IMO all organisms including humans metabolically, intuitively learn to like sources of nutrients and energy. This sense is poorly understood or appreciated. I think it applies to sunlight D as well as oranges C.

    Quote Originally Posted by DrRocket
    I would make an educated guess that absorptive specks would be better.

    That is because once light hits an absorptive speck the game is over.

    When light hits a reflective speck it goes somewhere else. If it goes away from the body then the game is over. If it reflects to some other speck it could go out or penetrate further. So now you have a random walk of the light ray going on and some of the reflected light will penetrate.

    So, if you have absorptive specks uniformly distributed that intercepted any incoming light ray, there would be zero penetration, but if the specks uniformly distributed and randomly oriented, some would penetrate. The degree of penetratin with the reflective specks would depend on the depth of the layer.

    A mirror skin would be a different thing. In that case all of the light would be reflected away and you would have no penetration, and with no absorption no heating either.
    Thank you.

    Stanley514, how's that? Given that skin must be (microscopically) rough, porous, and translucent, smooth mirroring just isn't possible.
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    When light hits a reflective speck it goes somewhere else. If it goes away from the body then the game is over. If it reflects to some other speck it could go out or penetrate further. So now you have a random walk of the light ray going on and some of the reflected light will penetrate.
    Interesting play of imagination.

    Well,I don't know why but human hair seems do not afraid any malignant melanomas.I didn't hear that
    some person would get cancer of hair.So what prevented nature to make skin rather like hair?And why
    people of the South have dark hair?Maybe it's better to have curly white hair on all the body?Or some scales?
    Cockroaches are very protected from many kinds of radiation
    and I guess not due to melanin.So there exist many methods of radiation protection invented by nature and
    we just need to recognize that black skin is not best of them.
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  33. #32  
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    Hair's dead. It can't get cancer.

    What's the point here? Evolution produced dark skin not because it was the best of all possible responses to increases sun exposure, but because it was the best of all possibilities within the range of human genetics at the time. Since then evolutionary pressures have changed so that it's neither a benefit nor a drawback, so it persists as a possibility, but not as the only one.
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  34. #33  
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    Quote Originally Posted by Stanley514
    So what prevented nature to make skin rather like hair?
    Skin is rather like hair, in that where the sun does shine it's dead. Now, everybody needs some light to filter through, so we get our vitamin D; the strength of one's filter will depend on local exposure. You may even adjust your filter with the seasons. Seems like an effective adaptation to me.
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    Why vitamin D production depends on sunlight?Humans are not vegetables and shoudn't depend on photosynthesis.

    If you very like idea of UV absorption,I think it would be very easy for nature create for you another white layer of dead white skin just on top of your black skin.Everithing that would not be reflected will be absorbed by black deeper layer
    of skin.Also it would be fine to have white curly hair on all the body just like Merinos.Wouldn't it be beauty?
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  36. #35  
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    Humans (used to) get lots of sunlight, and sunlight has energy. If we can use that energy to make something useful that would otherwise need to be obtained some other way, that's an evolutionary advantage. That we didn't end up with some other set of UV adaptations (which might be even better in some objective terms) is purely coincidence.

    Anyway, this topic's 3 months old. You might want to pay attention to the last post date when searching (though I guess as the OP, you have some right to reopen it).
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    I think that platinum or gold could be a good reflector for uv....no ionic reactions and no oxides ...perhaps some heat. I'm building a reflector coated with gold for uvc like a infinite mirror to increase the uv. And the power supply (maximum output 500w)will work with high frequency to adjust power and feedback . This is the mix of maser and laser teory. I need some help in my project.
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  38. #37  
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    I think nothing can effectively reflect UV. From standpoint of physic: UV oscillate very quickly, and thus electrons in metals can't catch up with such oscillation, and so metals cannot reflect it. An example of metals is iron & aluminium and other shiny things, but you could block UV by absorbing it (atomically) and then re-emmit it in different direction (scatter it); which is is NOT reflection (it is absorbtion and scattering).

    But,
    Melanin is much superior because it can convert UV rays into something else (it can convert it into heat or moving-ions, and do not re-emmit it), and it also absorb Gamma rays too. So melanin is the most advance material for radioprotection. I believe soon it will be used to power nano device too!
    Last edited by msafwan; September 30th, 2011 at 10:56 AM.
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  39. #38 Sigh. Silly humans. 
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    I like this thread, but I can't let some of these things go without saying something. A lot of the things said here could have been avoided with a little common sense. I'll address them. • Black skin resists direct sunlight better than white skin. This is a fact. It is beyond question.• The color black absorbs more light and white reflects more. This is also a fact. -- Many of you took this to mean that, in theory, if a white person and a black person were naked in a desert, the white person should survive better. Clearly this is false. It ignores a lot of things to even consider it. Melanin exists in the skin.. and the skin is the OUTER MOST LAYER of our cells, developed as a barrier against things that would otherwise be harmful (which would be pretty much everything). Yes melanin does heat up faster, but skin expresses heat outward when necessary. Meaning that the heat generation of darker skin is mitigated and negligeable (this being separate from the heat of your environment trying to cook you, which can't be mitigated as easily. Thus meaning a black person and a white person in an oven would cook the same, but a black person and a white person under UV lamps would show wildly different effects. Pardon the metaphor.) Does white skin reflect more light? Yes. Does it reflect ALL light? No. The smaller amount of light striking white skin, unabsorbed by melanin, would rapidly destroy the skin barrier, lessening the protection from the "outside".. so sure, the white guy in the desert would not heat up as fast from the sun, but his defenses would rapidly be pulverized (like shooting a stone "reflective" wall with a machine gun versus shooting a "force absorbent" pool of water).Remember that the UV light is dangerous because it damages DNA, thus causing cell mutation. If it is absorbed, by, say, melanin, before it reaches DNA, you're safer. Melanin IS damaged by this, the same way paint is bleached by the sun. The body just regenerates it. So why not spare ourselves the trouble and just not evolve for be pure alabaster white, or better yet reflective chrome? Because we do need to absorb other things the sun puts out. Like the aforementioned Vitamin D which we get mostly and best from solar light, and it takes much more energy to become reflective (this is why there are NO chrome animals or bugs anywhere on Earth's surface, but TONS of black creatures. Things get a little weird when you go, say, underwater.)Hope that helped.
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  40. #39  
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    Quote Originally Posted by Nookleer View Post
    I like this thread, but I can't let some of these things go without saying something. ..
    And the prize for resurrection of a necro-thread goes to you.
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    Perhaps a yellow ribbon.

    So why not spare ourselves the trouble and just not evolve for be pure alabaster white, or better yet reflective chrome? Because we do need to absorb other things the sun puts out.

    And there are limits to evolution as well as other evolved strategies we see in humans such as thin blond and red hair to absorb even more sunlight in foggy high latitudes.
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  42. #41 Materials uv 
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    Quote Originally Posted by stanley514 View Post
    could somebody explain why black color is good
    ultraviolet reflector when it does absorb if and visible
    light well?where could i find a chart that shows reflectivity of different colors to uv?
    there's no material that can reflect u.v . Everything absorbs u.v
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    What we describe as ultraviolet covers a range of wavlengths. I don't know if there is a generally accepted defintion of what wavelength range corresponds to ultraviolet, but I would expect it to include anything between 400nm and 100nm. The reflectivity of any surface would usually depend on where within this range one is thinking of - something that reflects well at 400nm might be awful at 100nm. The notion that "black" surfaces reflect ultraviolet well seems a bit of a contradiction. One could define a black surface as one which doesn't reflect any radiation regardless of its wavelength.
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