Working in a community of curious physicists, I was intrigued yesterday to hear a story about the steps taken to avoid the counterfeiting of Euro notes.
Apparently, under ultra-violet light, a Euro note will glisten in several different colours, and the precise colours produced can be used to check that the notes are not forged. Some physicists study light, and they use instruments called spectrometers to break light down into it constituent parts to analyse the type of atoms that are present. All atoms have characteristic light patterns, called spectra, and with the right type of expertise you can identify them. This is how astronomers determine the materials in distant stars or indeed in the sun.
|Europium used to prevent counterfeiting|
In the particular case of the Euro notes, the light is produced by a process called fluorescence. The energy from ultra-violet light is used to excite the fluorescing atoms, and then for a short time the atoms radiate their energy again, but at different colours. Some atoms fluoresce for only a short time and others for much longer.
The rather beautiful finding that surprised me yesterday was that Euro notes use an element called Europium in a special (and undetermined) formulation. There is something rather poetic about this isn't there?
I only hope that USA does not follow the same poetry for American dollar bills. Unlike the two stable isotopes of Europium (with masses 151 and 153), all the isotopes of Americium are radioactive and it is not a material that you would want to have in your pocket.
Small note: It has been suggested that it might be illegal to investigate the sources of the other colours in too much detail! Genuine scientific curiosity could be misinterpreted as conspiracy!