The bumps and flats are arranged in a continuous track, called a spiral path, which measures about 0.5 microns (millionths of a meter) across and 5 km long. To read this information, a CD player passes a laser beam over the track. When the laser passes over a flat area on the track, the beam is reflected directly to an optical sensor on the laser assembly. The CD player interprets this as a 1. However, when the beam passes over a bump, the light is bounced away from the optical sensor and the CD player recognizes this as a 0. Consequently, conventional CDs have remained a "read only" storage medium for the average consumer and computer users are starting to become very fed up with the limited capacity of floppy disks. In response to great demand of higher capacity disks, electronics manufacturers introduced the CD-R.
CD-recordable discs open new doors to the idea of storage mediums. Computer users would be able to store data ranging from 640 to 700 MB. Unlike regular compact discs, CD-Rs don't have any bumps or flat areas at all. Instead, they have a smooth reflective metal layer, which rests on top of a layer of photosensitive dye. When the disc is blank, the dye is translucent allowing light to shine through and reflect off the metal surface. When you heat the dye layer with concentrated light of a particular frequency and intensity, the dye turns opaque; it darkens to the point that light can't pass through. This is the job of the CD-burner. The CD burner has a moving laser assembly, just like an ordinary CD player, but in addition to the standard "read laser," it has a "write laser." The write laser is more powerful than the read laser, so it interacts with the disc differently. It does this by altering the surface instead of just bouncing light off it. At the bottom plastic layer of a "write laser" are grooves which are pre-pressed into it, to guide the laser along the correct path.
