Perhaps everyone has already seen it, perhaps not.
With a simple sheet of printer paper, which is almost always coated in optical whiteners that fluoresce under UV light to make it appear whiter, you can show the normally invisible UV spectrum lines of a mercury vapor lamp:
https://youtu.be/y8in-H5ChnwThe background/projection screen is painted wood, which does not show the UV lines. When i introduce the sheet of paper, they appear dim, but visible. Their dimness might be caused by the multiple soda lime glass lenses that each attenuate the UV even further.
Adding fluorescent dye (like dayglo markers) to the paper makes the effect more apparent, but also changes the color of the line.
The setup is shown in the picture. The Narva lamp is an ovoid glass one, encased in a sheet metal can with one opening it to direct the light. This is how they were sold by the supply houses of scientific equipment. After that comes a lens, a slit, another lens, a diffraction grating and yet another lens.
For this setup to work, it's important that no other light reaches the screen because it will wash it out. A HQL/HPL without the metal canister-like cover will wash out the very pretty spectrum lines.
This setup is one that was used for light spectrum demonstrations at work before i started working there, but the whole set is still complete and functional. These days, we use some very high quality spectroscopes (
https://www.patonhawksley.com/product-page/benchtop-spectroscope) we hand out to the observers. The resolution is so good that you can see that the yellow line in the mercury spectrum isn't just one line, but two lines, very close together.
The same lamp is also used by the Chemistry department to decolorize bromine solutions in some chemical reaction. I've handed them a much more easy to acquire PL-style UVC lamp for that purpose, because obviously clear HID mercury arc lamps have gotten hard to get - though if you pay 250 euro or whatever, you can still get them at the large scientific instrument supply houses for as long as stock lasts.
It should last for thousands of hours, but considering it's always running very hot in that metal can, i want to avoid putting unnecessary wear on it.
