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!