Uranium Glass

Uranium compounds have been used to colour glass since ancient times. A mosaic found in a Roman villa dated to about 79 AD used yellow glass containing 1% uranium oxide. Between 1880 and the 1920s, uranium glass was quite popular, not only because of its interesting yellow-green colour, but because it fluoresces in ultra-violet light as shown in the image of a vase below.
The image on the left hand side shows the vase under normal daylight, while the image on the right shows the same vase in the dark exposed to UV light.

Why does uranium glass glow green in the dark like this?
Well, it has nothing to do with the radioactivity of uranium isotopes. The green glow does not occur because of changes within the nucleus of uranium atoms, it occurs because of changes in the energy of electrons surrounding the nucleus.
A ground state electron in a uranium atom absorbs the energy of a photon  of ultra-violet light causing the electron to jump to a higher energy level known as the excited state. This new excited state electron configuration is not stable, so, the electron falls back down to to the ground state energy level, which is a lower energy state, by losing energy which it does by emitting a photon of light. Some of the energy of the original photon used to excite an electron is dispersed as molecular vibration and heat, so the energy of the photon emitted when the excited electron falls back to the ground state is less than the energy originally absorbed.
That is: E = hν/λ
where E = energy, h = Planck's constant, ν = speed of light , λ = wavelength of light
If the speed of light is a constant, then E ∝ 1/λ or Eλ = a constant
in other words, the more energy the photon of light has the shorter its wavelength is.
In the case of uranium glass, the absorbed photon of ultra-violet light has a wavelength of about 300 nm, while the emitted photon of green light has a wavelength of about 550 nm.
The wavelength of the emitted photon of light corresponds to the green part of the visible spectrum.
The emission of visible light in this way is known as fluorescence.

Further Reading:
Isotopes
Radioactivity: Properties and Uses
Emission Spectra
Flame Tests

Suggested Study Questions

1. Uranium is found in nature as one of three isotopes, uranium-234, uranium-235 and uranium-238. Explain how atoms of each isotope are:
• the same
• different
2. Atoms of neptunium-234, neptunium-235 and neptunium-238 have been synthesized. Explain the similarities and differences between each of the following pairs of atoms:
   • neptunium-234 and uranium-234
   • neptunium-235 and uranium-235
   • neptunium-238 and uranium-238
3. The value for Planck's constant is 6.626 070 040 x 10^-34 J s and the speed of light is given as 300 000 000 m / s. Calculate the energy of each of the following photons of light:
• ultra-violet light, λ = 300 nm
• green light, λ = 550 nm
4. Using the results of question 3, compare the energy of the photons used to excite uranium atoms and the energy of the photons emitted by uranium atoms and explain the difference.
5. Explain why the same wavelength of light is always emitted when these uranium atoms are exposed to ultra-violet light.
6. Would you expect the uranium glass to fluoresce if it is exposed to infra-red light rather than to ultra-violet light? Explain your answer.
7. Optical brighteners are often used in laundry detergents to make your old, yellowy-looking white clothes look whiter. Typically these compounds absorb ultra-violet light with a wavelength of around 300 nm and emit light with a wavelength of around 450 nm.
   • What colour is the light emitted?
   • What is the energy of the emitted light?
   • Why is the energy of the light emitted not the same as the energy absorbed when the incoming photon hits the "optical brightening compound"?
   • Would you expect a laundry detergent containing "optical brightening compounds" to glow in the dark when exposed to UV light? Explain your answer.