The Chicago River was an Irish green on Saturday March 14, 2009 for St. Patrick’s Day today:
But what exactly makes the Chicago River GO GREEN? Fluorescein.
Atoms and molecules (which are simply bound collections of two or more atoms) can absorb light (energy). Atoms are made up of protons, neutrons, and electrons, and each chemical element has a specific number of them (that’s what makes them different). Protons and neutrons are heavier than electrons, and we often picture an atom as a miniature solar system, with the heavy particles at the center (the nucleus) and the electrons whizzing around in specific orbits like planets. So for example, the hydrogen atom only contains one proton and one electron and therefore is the simplest element in the Universe. If left undisturbed, the hydrogen atom likes to bind its electron as tightly as it can, and so the electron is in the lowest energy level, called the ground state. However, if the atom is immersed in a beam of light, sooner or later the atom will encounter a photon with an energy that is just the right amount to jump the electron up to the next higher energy level. The photon gets absorbed, and is gone from the beam of light. Since the absorbed photon had a specific energy, this absorption occurs at a specific wavelength in the spectrum. Now the hydrogen atom is in an excited state. If no other photons are absorbed by the atom, the electron will eventually drop back down to the lower energy ground state. However, the atom has to lose energy to do this, and so it releases a photon of the same energy as the one it absorbed. This is called emission because a photon of light is emitted by the atom, again at a very specific wavelength.
Lots of molecules absorb light. For example, a red marker and pink highlighter both contain dyes that absorb green light. The difference between the marker and the highlighter, however, is that if the highlighter’s ink absorbs a photon, 10 times out of 100, it will be emitted as light while if the marker’s ink absorbs a photon, 0.1 times out of 100, it will be emitted as light. So now back to fluorescein in the Chicago River on St. Patrick’s Day. Fluorescein is strongly fluorescent: if it absorbs a photon, 97 times out of 100, it will be emitted as light. Consequently, fluorescein was used in the Chicago River in 1962 by a plumber to detect river pollution, and it is now used every St. Patrick’s Day to dye the River Green. Fluorescein is also used in biology to fluorescently tag molecules.
Chicago really goes GREEN on St. Patrick’s Day (from the river to the beer to the attire). Happy St. Paddy’s Day from Chicago (and the engineer on my Metra train this morning)!
The engineer on my Metra train this morning!
-Heather
Posted by heatherselby 
