Why does Pyrex glow orange under UV? In a Physics lesson today our teacher performed a demonstration to show how quinine in tonic water glows blue when under UV light. He showed us the same demonstration using water in a Pyrex beaker, and with just the Pyrex beaker itself in order to compare the effects.
Under UV light the Pyrex beaker glowed orange. Why did this happen?
And more generally, what determines the colour of visible light emitted by the object under UV light?
If you were able to shed some light (:D) on this it would be appreciated.
 A: The manufacturer's of Pyrex glasses use a special technique to make laboratory glasswares. They call it amber colouring. They do this by spraying a special mixtureon the outer layer of the glassware whose exact combination is not known, and not needed for present purposes. All we need to know is it's properties and why do they do it.
Why do they do it? Well,these glasswares are used for various photo-sensitive work. Like people keep some chemicals on them which actually breaks into other substances in presence of UV rays. Ambering helps resist this. The transmission curve is like:

So you can see that UV rays do not pass through the glass at all.
Now I guess the UV ray generator your teacher used obviously had generated some(very small) amount of visible light spectra. Since the short wavelengths were absorbed by the coating, all you can see was the orange-ish spectrum.
Source of info and picture: http://www.duran-group.com/en/about-duran/duran-properties/optical-properties-of-duran.html
A: The orange light is very likely sodium D lines. 
Sodium is present in glass, and emits light when excited. Electrons are easily promoted from the 3s to 3p orbitals. When they decay back to 3s, they emit light at 390.0 and 390.6 nm. The reason for two lines is spin-orbit coupling. Sometimes the spin angular momentum of an electron is aligned with the orbital angular momentum, and sometimes opposite. There is a slight energy difference. 
Sodium can be excited a variety of ways. Heating does it. Sodium vapor lamps do it with an electrical discharge. An electric pickle does it with a current. 
Sodium can also flouresce. Here is a pay walled paper describing it. 
I can only describe in general terms why a given substance has a given color. When an electron drops from one orbital to another, it loses energy. If the energy is emitted as light, the frequency and wavelength are related to the energy loss by $E = h \nu = hc/\lambda$. The frequency and wavelength determine the color. For sodium, the color is in the middle of the visible spectrum.
