The CD: A Sandwich That Bytes
A CD is a four-layer sandwich . . . a layer of plastic, covered by a fine layer of aluminum, followed by a thin layer of acrylic (another type of plastic) and, finally, the label. Voilà! A "CD sandwich!"

Much like a blind person reading Braille, the CD player’s job is to read the data bumps and convert them in to signals that can be understood by the system’s amplifier. Here’s what happens: First, the laser focuses on the track of data bumps. Next it shoots the laser beam through the clear (polycarbonate) layer so that it reflects off the aluminum layer. Finally, the reflected light activates a special on/off switch, called an opto-electronic switch. Light reflecting off the “flats” (the parts where there are no bumps) turns the switch “on” and light reflecting off of the “bumps” scatters and leaves the switch off.

The laser bounces off flats and turns the optical pickup on, but leaves it off when it bounces off the bumps.

CD Rainbow If you had a completely blank CD, one without the spiral of bumps, it would be a perfect mirror. The bumps are what give a CD its rainbow colors! Have you ever noticed the rainbow of colors when you move the shiny side of a CD back and forth? What makes this happen? Here’s a quick explanation. CDs without data tracks are perfect mirrors. But the data tracks cause microscopic changes to the surface of the CD and make it act like a prism. Light rays bend (diffract) when they reflect off of the surface and separate into different wavelengths. We see each wavelength as a different color.

Voltage
In the CD player, each string of 1s and 0s corresponds to an electrical signal (a voltage). The DAC (digital-to-analog converter) turns the numbers into voltages. The voltages change 44,000 times per second! The amplifier sends the voltages to the speakers where they turn into a series of sounds. The graphic below shows how ones and zeros turn into voltages for the amplifier.

Voltage samples (base-10 value) from 4-bit codes

There is a good bit of computer technology inside your CD player. It converts the ones and zeros into understandable data blocks. These data blocks then go to the digital-to-analog converter (DAC), on to the amplifier and finally to the speakers. And all this is done in only nanoseconds!