Well anyway, atoms can combine in different ways to form molecules. When this happens, the electron orbitals from all the individual atoms fuse into new orbitals called molecular orbitals. When the molecule is just hanging out (i.e. when the light is off), those electrons are generally in the lowest-energy orbitals possible. However, when a particle of light (a photon) with the right energy interacts with the electrons, they can absorb the energy in the photon and jump up to a higher-energy orbital. The molecule is in what is called an excited state. However, this excited state is unstable. Think of it as sort of like a pencil balanced on its tip. Maybe the molecule can exist in that state for a split second, but it wants to get back to its ground state. In some molecules, the electron just falls back to its low-energy state and the energy from the light is lost as heat. But in some special molecules (like the molecules of the fluorescein that I brought you), something really cool happens where the molecule emits a new particle of light when the electron falls from its excited state back down to its low-energy state. This is the light that you observe as fluorescence. The new photon of light is a different color than the original photon of light because some of the energy is typically lost in the process of kicking the electron up to a high-energy state and letting it fall back down again. I hope that wasn’t too unclear! Let me know if you have any more questions!
Note: 1 nanosecond = 1/1,000,000,000 second
1 picosecond = 1/1,000,000,000,000 second
1 femtosecond = 1/1,000,000,000,000,000 second!!!!!!
What's really crazy is that people have figured out ways to observe things this fast! One of the people who figured out how to do this won the Nobel Prize. See: http://nobelprize.org/nobel_prizes/chemistry/laureates/1999/illpres/
