This post begins what will probably be a three to four-part endeavor to explain the basics of how lasers work. Comments and questions welcome, I'll try to explain this all as simply I can. Additional details will be parenthesized and are only supplemental material not necessary for the understanding of each article, aka feel free to skip them.
PART I: FLUORESCENCE
To develop the background necessary for later parts, I will avoid lasers here and discuss the basics of energy levels and emission. Atoms and molecules have a discrete set of electronic energy levels (which may be obtained by the solution of Schrodinger's eigenvalue equation for the electronic degrees of freedom - if we do the same for the nuclear degrees of freedom we can obtain rovibrational energy levels, but from here on out, we'll ignore them). These energy levels can be ordered, with the lowest one being called the 'ground state.'
When left alone (i.e. thermal equilibrium), a majority of the particles will be in the ground state (with the ratio of particles in each state equal to the Boltzmann factor for the energy difference between the two states). If we add energy to the system, perhaps in the form of light, we can excite the ground state atoms into an 'excited state.' But the energy we add has to be exactly equal to the energy gap between the two states. Once our atom is in an excited state, it can very quickly relax back down to the ground state via the emission of a photon with an energy equal to the energy gap. In principle, this describes fluorescence: we shine light on something and it immediately starts giving off light. As soon as we turn off our light, fluorescence will stop because all of the excited state atoms will immediately return to the ground state.
This is basically how glowsticks work. Although rather than using light for the excitation, a chemical reaction occurs (when you snap the glow-stick, letting the chemicals mix) which releases energy, exciting a dye molecule. When the dye molecule relaxes back down to its ground state, it releases energy in the form of light. Once the glowstick runs out of chemicals, the reaction stops, and the glow-stick stops glowing. This is also when you cry and snap another glowstick to make yourself happy again.
Please ask questions, all 3 people who probably read this, and I'll do my best to answer them.
Next time - PART II: PHOSPHORESCENCE
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3 comments:
Are glowsticks a distant cousin of lasers? And, is it true that if you put a glowstick in the refrigerator it'll start glowing again?
Holy shit someone actually commented.
First off, of course glowsticks are a distant cousin of lasers. Few people know that a glowstick can be turned into a death ray with household baking soda.
And nay, freezing a glowstick will not make it start glowing again, quite the contrary. Reducing the temperature will slow down the reaction, prolonging the life of the glowstick. So you could stick it in the freezer, wait a while, take it out, and when it warms up it will start glowing again.
Thanks for your comment!
eigenvalues! (thats all i picked up)
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