How do CDs work

What is CD ?

• A compact disc is a thin, circular disc of metal and plastic about 12cm in diameter. It is made of three layers. Which is made from a plastic called polycarbonate. Sandwiched in the middle there is a thin layer of aluminum. Finally, on top of the aluminum, is a protective layer of plastic.
• The dull side usually has a label on it telling us what's on the CD, the shiny side is the important part. It's shiny so that a laser beam can bounce off the disc and read the information stored on it.

How does a CD Work?

• CDs are "burned" with a laser beam that makes bumps (called pits) into its surface. A bump represents the number zero, so every time the laser burns a bump into the disc, a zero is stored there. 
• The lack of a bump (which is a flat, un burned area on the disc, called a land) represents the number one.

Reading Device :

It is made up of three main components.

• Drive motor (which will spin the disk).
• Laser lens (it will shoot laser on to the disk and figure out whether there is a reflection coming back).
• Tracking mechanism (which will moves the laser lens from the inside of the disk to the outside of the disk at the right time).

• When we press play, an electric motor makes the disc rotate at high speed. The laser beam switches on and scans along a track, from the center of the CD to the outside.
• The laser flashes up onto the shiny (under) side of the CD, bouncing off the pattern of pits (bumps) and lands (flat areas) on the disc. The lands(1) reflect the laser light straight back, while the pits(0) scatter the light.
• Every time the light reflects back it detects and realizes it's seen a land(1), and sends a burst of electric current to an electronic circuit that generates the number one.
• When the light fails to reflect back, the photocell realizes there is no land there and doesn't register anything, so the electronic circuit generates the number zero.

How does a recordable CD (CD-R) work?

• In between the protective polycarbonate and the reflective aluminum, there's a layer of dye.
•  A CD-R writer has a higher-powered laser than normal, which generates heat when it strikes the disc, "burning" the dye and making a tiny black spot.
• Later, when a CD reader aims its laser at that spot, the light is completely absorbed and doesn't reflect back. This indicates that a zero ("0") is stored on the disc at that point. In places where the dye is unburned, the laser light reflects straight back again, indicating that a "1" is stored on the disc.

How does a rewritable CD (CD-RW) work?

• Instead of having a layer of dye, a CD-RW has a layer of metallic alloy that can exist in two different solid forms and change back and forth between them. It's called a phase-change or phase-shift material.
•  Sometimes it's crystalline, with its atoms/molecules arranged in orderly ways, so it's translucent and light can pass straight through it; other times, its atoms/molecules are jumbled up in a much more random and disorderly form called an amorphous solid, which is opaque and blocks light. 
• When a CD-RW laser hits this material, it changes tiny little areas of it back and forth between the crystalline and amorphous forms. When it creates a crystalline area, it's making part of the CD reflective and effectively writing a one ("1"); when it makes an amorphous area, it's making the CD non-reflective and writing a zero ("0").
•  Because this process can be repeated any number of times, we can write and rewrite a CD-RW pretty much as many times as we like.

Why DVD and Blu-ray can store more information ?

When we're writing computer data on discs with laser beams. We can store more on a DVD than a CD by using a laser beam that "writes smaller". And to read or write a Blu-ray disc, we use a laser to write even smaller still.