We Can Figure This Out.org
Virtual Lab: The Inner Workings of CD/DVD Drives
 University of Virginia
             
 
© 2003-Present, John C. Bean
 
But now things get more complicated. CDs and DVDs are not perfectly flat. Even if they were, they might not sit flat inside the drive.

So, as the disk’s surface goes up and down, it can move above and below the point where the laser comes to a focus. At those points the laser beam may be so broad that it spans multiple bits of information.

So we must have a way of moving the lens to keep the laser sharply focused on the disk's surface.
 
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