|Why do you hear different music coming from a compact disc when the laser of the CD player is just going around the same part of the CD over and over again? — KD, McMinnville, OR
The CD player's laser doesn't really go over the same part of the CD over and over again. As the disc turns, the laser slowly moves outward from the middle of the disc toward its edge. The laser beam is focused to an extremely small spot inside the disc and it is carefully following a tight spiral ridge in the aluminum layer inside. This ridge runs continuously from the center of the disc to its edge. With each revolution of the disc, the laser works its way outward by one more turn of the spiral. The ridge has interruptions in it every so often and it is this pattern of interruptions that contains the information needed to reproduce sound.