Light 15: Lasers

Welcome to the 2023 Advent Calendar, which this year is just a list of things that light up all pretty. Previous entries are here. If you enjoy this and want to encourage me to bang more things out on a keyboard, consider supporting my Patreon, or sending something off of my Christmas list. If you'd rather support my spoiled pets, their Ratmas list is here. If you want to spend money but not on me, you can direct your donations to Mainely Rat Rescue, who handles rescue and foster operations for rats, mice, gerbils, guinea pigs, and other small mammals in the New England area, or the MSPCA, where my critters got their medical care before I found a good exotics vet, and where I picked up Koda and Yogi. 

Enjoy your trip through the cavalcade of things that go blinky-blink in the dark!

We all know the Austin Powers joke about Dr Evil and his "sharks with frickin' laser beams", but did you know that for Dr Evil, frozen since the 1960s, that would have actually been cutting-edge technology? Completely true! The first laser was only built in 1960, two years after the concept paper describing it was published in 1958. Before that, Buck Rogers and his ilk had to settle for mere "ray guns" without a single shred of pseudoscience to explain them.

Though technically LASER is an anacronym, people started using "laser" as a noun almost immediately, mainly because they had no idea what "Light Amplification by Stimulated Emission of Radiation" meant. The short explanation is that to make a laser, you apply energy to a substance until photons shoot out. The difference between a laser and a regular flashlight is that in a laser, the inside of the tube is mirrored so the photos bounce back and forth, whacking into more and more random molecules of the material that emitted them in the first place to jar more photons loose. One end of the tube is only partially mirrored, so eventually a few photons bounce off the back at just the right angle to get through, and voilá! You have a laser beam. 

(Because all of the photos are being emitted by the same substance, lasers are generally one coherent wavelength of light, but this isn't actually part of the definition. You can make lasers that emit more than one color of light, if you try real hard, and you can make ordinary lights that are all one wavelength, like sodium streetlamps.)

Lasers are used in a hundred bazillion things, their most useful feature being that the light beam is collimated, which means that the photons all follow paths that are nearly parallel. This keeps the beam small and tight, enabling the laser to be focused on a very small, very precise spot, anywhere from the mouth of the laser to thousands miles away. It makes them equally perfect for reading the tiny pits and lands of a CD, and for hitting retroreflectors left on the goddamn moon.

Though industrial lasers can be huge unwieldly contraptions, involving gas canisters and monster cooling systems, the handheld kind so beloved of cats is driven by a mere light-emitting diode. The first color of diode available as a (huge, expensive, probably finicky) pointer in the 1980s was a deep red 650nm; almost 20 years would pass before someone managed to get a green 532nm diode cheap enough to offer those. Most recently, some 405nm "blue" (actually more of a vioet) diodes were diverted from the manufacture of Blu-Ray players to use in the most hazardous pointers yet. A decade ago those were about $60, but judging from the Amazon listing, you can now get a somewhat questionable facsimile of one for about $3.33/ea.