Glowing Glass


Concepts Shown:

recombination, luminescence


ultraviolet light, glass rod of any size or shape, containing uranium oxides. Glass rod should be prepared by a professional glassblower. The uranium content is extremely small and is very safe. The rod emits 0.5 millirems of beta rays, which is very small compared to allowable limits.


Turn off lights and place glass in front of ultraviolet light.


Recombination: Energy in the form of a photon from the ultraviolet light moves an electron from the valence into the conduction band. E (energy) � n (conducting electron) + p (hole in valence band). This forms an electron-hole pair. Materials prefer lower energy states and the pair recombines: n + p � E. The UV light emits photons at a certain energy level such that visible light is emitted upon recombination. [eq]. Recombination Rates: Every material has a particular interval in which every electron has the same probability of recombining. The formula which determines this rate is the following: N = No exp(-t/T) No = Number of electrons in the conduction band at a particular time (i.e. time after light has been turned off) t = Time after light has been turned off. T = Recombination time or relaxation time. Characteristic of material. N = Number of remaining electrons. Luminescence: Luminescence is the emission of light as a result of recombination. Using light to cause luminescence is called electroluminescence. Photons from the black light cause electrons to jump into the conduction band. When they recombine energy is given off as light that students can see. Decay Curve: The number of activated electrons determines the recombination rate. The following formula determines the intensity (I) of luminescence: I = Io/exp(t/T) I = Intensity after time, t Io = Original intensity T = Recombination time. [eq]. Comments: This demonstration is very easy to do. It is important that the uranium glass is prepared by a professional glassblower since uranium is radioactive.


Marcia Muller

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