I was carrying out a photoelectric effect experiment when I realised that the $365$ nm line in the mercury spectrum was surprisingly visible when shone onto a piece of paper. This lies in the UV spectrum, and it is not visible directly. However, I could see it clear as day when it was shone onto a piece of white paper.

My conclusion is that the particular reflection happening in this scenario is causing the wavelength to change. (Either the wavelength is "spreading out", so that it is not just $365$ nm, but some range of values centred at $365$, or the overall wavelength has shifted up.)

I've tried to figure out if there is a property of diffuse reflections that would result in a change in wavelength, but I haven't come to any conclusion.

An explanation of this effect would be greatly appreciated, thank you!

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    $\begingroup$ I do a lot of work with mercury lamps. I can tell you for sure that there are 2 UV spectral lines that should become visible on white paper; not just one $\endgroup$ – Jim May 24 '17 at 12:05
  • $\begingroup$ Because it is not UV anymore when it is... "reflected"... From the paper. It is a form of luminescence, not a reflection, as you can see by noticing the emission doesn't obey the Snell law. $\endgroup$ – Vendetta May 31 '17 at 19:20

Paper sometimes contains a fluorescer to make it look whiter. I would guess that the light you are seeing is coming from fluorescence induced by the UV light.

In fluorescence some of the original light energy is lost to lattice vibrations so the emitted light has a longer wavelength than the absorbed light.

You can see the shift in wavelength if you use a violet light source (such as a simple laser pointer). On dark/non-fluorescent surfaces, you see the violet light reflected. However, on fluorescent surfaces, like white paper, you will see blue light. While not as cool as the effect with a mercury lamp where bands appear out of nowhere, it does visibly showcase one light shifting to a longer wavelength of light due to fluorescence.

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    $\begingroup$ High quality business cards make great UV laser viewers (and generally cheaper than the viewers sold through optics suppliers). $\endgroup$ – Jon Custer May 24 '17 at 12:36
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    $\begingroup$ Also, if you have a glowing-in-the-darkness toys/objects like this one, you can see them begin to glow with their (usually green) color when exposed to the UV light. That would be another type of photoluminescence: phosphorescence. $\endgroup$ – Ruslan May 24 '17 at 17:12
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    $\begingroup$ This is a related and fun question: Why does laser light not affect glowing materials? $\endgroup$ – John Rennie May 24 '17 at 17:20
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    $\begingroup$ For white paper, it almost always contains fluorescent dyes, not just sometimes. Try to find white paper stock that doesn't contain this dye. They don't usually bother putting it in toilet paper, but otherwise it's almost everywhere. I think it became common in printed books around the 1990's. That's just a guess by taking a quick scan of my library with a UV light. $\endgroup$ – Noah Spurrier May 25 '17 at 12:02
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    $\begingroup$ If you've ever wondered why books look so glaringly bright while you're reading them in the sun, this is why. $\endgroup$ – einnocent May 25 '17 at 19:57

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