Today’s Wonder of the Day was inspired by Belinda from South Africa. Belinda Wonders, “how do CDs work?” Thanks for WONDERing with us, Belinda!
Are your parents and relatives on the cutting edge when it comes to technology? For example, when they want to listen to music, do they download or stream music online? Or do they instead reach for one of those small, silver discs?
How we listen to music has changed a lot over the years. Your parents and grandparents have probably seen an evolution of music that saw media go from record albums to reel-to-reel tapes to eight-track tapes to cassette tapes to CDs.
When CDs (short for "compact discs") were invented, they revolutionized the music industry. It only took digital CDs a few years to conquer the market, sending analog cassette tapes along the same route to obscurity taken by their predecessors. So exactly how do these shiny silver discs work, and what makes them so great?
CDs, while not as popular as they once were thanks to content that's downloadable and streamable from the Internet, are still frequently used to store music and large quantities of digital data. The fact that they store data digitally is one of the reasons they became so popular. Compared to older forms of media, CDs provide music quality that's crisp and crystal clear.
CDs are also built to last. Cassette tapes, record albums, and many older types of media were prone to damage from scratches, heat, and everyday wear and tear. CDs, however, can withstand much more punishment without affecting the quality of playback.
Their resiliency is a product of their construction. The majority of a CD consists of layers of a clear plastic called polycarbonate. Sandwiched between these layers of plastic is a thin layer of aluminum, which is coated with a protective layer of acrylic lacquer.
The data on a CD consists of an extremely-long, continuous, single spiral of microscopic bumps, starting at the center of the disc and circling out to the outer edge of the disc. If you could unravel and stretch out the data track from a CD, it would be half of a millionth of a meter (0.5 microns) wide and over 3.5 miles long!
So how does a CD player or computer convert those billions of bumps into music? It takes a very precise laser beam focused on the data track as the CD spins rapidly around. The laser beam passes through the polycarbonate layer and reflects off the aluminum layer. As it does so, a sensor detects changes in the laser beam's reflection, since the bumps cause the laser to reflect differently.
Sophisticated computer technology converts all those reflections into a stream of digital information that can then be interpreted as music, data, or whatever it happens to be. This all happens with incredible speed as the CD is played. Isn't technology awesome?