What’s the application? CD and DVD players use lasers to read (and in some cases write) digital information from convenient plastic disks.
What problem(s) is it the solution to? 1) “How do we store a large amount of digital information in a convenient and stable fashion?” 2) “How do we make everybody buy the White Album a second time?”
How does it work? The optics at the core of a CD player are very simple, and illustrated in this graphic that I lifted from the excellent explanation at HyperPhysics:
Light from a diode laser is collimated and then focused down onto the surface of the CD. The CD is covered with a reflective material with digital information imprinted on it as a series of spots that reflect more or less light. In the original CD’s, this is done with physical pits in an aluminum-coated layer sandwiched in the CD itself, hence “pits” in the figure. Recordable disks use a dye layer in front of a reflective layer, and a slightly higher laser power in the record mode that changes the reflectivity of the dye to make the same sort of pattern of bright and dark spots.
The light reflected off the spinning disk is directed onto a photodiode, and recorded as a series of 1′s and 0′s, which reproduces the digital data that was stored on the disk in the first place.
That’s pretty much it. There are some fiddly optics involved in keeping the beam on target, and making sure that only the reflected light you want gets read out, but those are details. The key feature is the light bouncing off the patterned disk.
Why are lasers essential? You need a bright source of light of a single wavelength that you can focus down to a small spot. the cheapest and easiest way to do that is with a laser.
Why is it cool? At the time of its invention, the CD was a dramatic leap forward in the storage density possible in portable media. And as there is no physical contact between the patterned surface and the reader, they’re extremely stable and damage-resistant.
The higher audio quality possible with CD’s (according to everybody but crazy audiophiles) drove a huge boom for the music industry, as people bought new copies of music on CD. It also started the transition to digital music, and thus paved the way for the modern world of downloadable MP3′s.
Why isn’t it cool enough? Magnetic and flash technologies have surpassed the data storage capacity of optical media (the size per bit is set by the wavelength of the laser, more or less), so CD’s and DVD’s are becoming less essential.
The vast profits reaped by the music industry turned out to be an unsustainable windfall, and turned record company executives into even bigger dicks than they were before the CD transition.