Yttrium2
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UV lights
What makes UV lights light up secret ink? Will any inks be revealed by the typical UV light? What other lights can expose hidden messages? How come
the UV from the sons rays does not expose the hidden inks?
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Yttrium2
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Light causes photon emission, I forgot the details, and am away from my desk 
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Fantasma4500
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well light is simply when all colours, except for one specific wavelenght is absorbed, thereby seemingly BEING the colour, but if you view a piece of
green plastic in nightvision, where only a very weak infrared lamp will be active, you wont really see it shining up green, if you ignore the common
green nightvision colour
i recall looking into UV colours, and its kind of more abstract, but there is this guy who got some neat videos, and he went a bit around it in this
video
https://www.youtube.com/watch?v=62pwykA_huU
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Sulaiman
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it is due to fluorescence https://en.wikipedia.org/wiki/Fluorescence
most substances do not exhibit fluorescence so most inks will not fluoresce
special (relatively expensive) fluorescent pigments have to be added.
fluorescent inks DO fluoresce under sunlight
but the fluorescence is MUCH less bright than the colours reflected by visible light (ROYGBIV) so you do not notice it.
P.S. many laundry detergents have fluorescent compounds added
so that whites are more brilliant (blue) in sunlight.
Also lots of green, yellow and orange plastics fluoresce.
I have a small u.v. torch and many times I have gone around the house checking what fluoresces.
[Edited on 26-8-2015 by Sulaiman]
[Edited on 26-8-2015 by Sulaiman]
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