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The common principle for optical discs is the reflection of laser in interaction with the polycarbonate data surface. The change in height between pits and lands results in a difference in the way the light is reflected. By measuring the intensity change with a photo-diode, the data can be read from the disc. 
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Each string of 1s and 0s corresponds to an electrical signal (a voltage). The DAC (digital-to-analog converter) turns the numbers into voltages.