# 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.

• It seems to me that the solution to this problem is as follows: the molecules absorb light at some frequency in the UV spectrum, becoming excited. Then, after some time there is spontaneous emission of light of a frequency that makes it appear orange to the human eye. Now, the gory details of this may be quite complicated and I won't claim to know them in this case, but this is the general background idea.
– Danu
Commented May 3, 2014 at 10:24
• The phenomenon is called fluorescence. I didn't think Pyrex (aka borosilicate glass) had any significant fluorescence, so I don't know why your empty beaker was producing orange light. Commented May 3, 2014 at 10:56
• Just a thought: Pyrex manufactured in the last 20-30 yrs may not be borosilicate (mfr went for cheaper stuff). Commented May 3, 2014 at 12:06
• When you shine a $UV$ Light you give electron's in atoms energy to go to next energy state (Energy of any Photon is described using this equation $E=hf$ where $h$ is Plank's constant and $f$ is frequency), when electron 'relaxes' to ground state it emits light with certain frequency ($f=\frac{\Delta E}{h}$). This phenomenon is called Flourescence (as @JohnRennie said).
– user21420
Commented May 3, 2014 at 17:02
• @GigiButbaia except you can't make bread with it (ya made a painfully common misspelling there :-) ) Commented May 3, 2014 at 21:25

## 2 Answers

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

• This could also be fluorescence at work, so you should support your scattering hypothesis with more than just a guess. Commented Nov 30, 2015 at 11:45
• As you mentioned "It's just a Hypothesis". It may or may not be what really is happening. But it's very much possible.
– Ari
Commented Dec 1, 2015 at 6:11
• Amber coloring is usually in the glass, not just a coating. When you break a beer bottle, the color is all the way through. Commented Dec 3, 2015 at 14:21
• So, when you say "it's just a hypothesis", you really mean "it's something that may or may not be what's happening, and I have no evidence to tell either way, so you shouldn't really pay that much attention to this"? Commented Dec 3, 2015 at 17:13
• @EmilioPisanty What I mean is: It's a very plausible scenario. So, you should give as much attention possible to it as you are giving to any other.
– Ari
Commented Dec 5, 2015 at 17:27

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.