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!
Best Answer
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.