I tried, that various photographic lenses how will behave under UV light and got quite interesting results. I borrowed a few lenses from a really good friend and started to do some experiments. Some of the lenses fluorescence under UV light with blue/green/yellow/ect. color.
Why do they glow under UV? Depending on the composition of the glass, it could easily contain mixtures a few rare earth elements, usually Lanthanum, Cerium
and sometimes Thorium
(yes, that radioactive thing at the bottom of the periodic table). These elements are added to the glass, since if they are present, the glass has a higher refractive index what means, that better optics could be made from them, with lower chromatic aberration. An example.
What is chromatic aberration? It’s also called achromatism or chromatic distortion, is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have different refractive indices for different wavelengths of light. The refractive index decreases with increasing wavelength.