Data storage on polycarbonate discs

Light from a solid-state laser is focussed onto the series of pits in the compact disc. The diffracted light is detected by a series of photocells. 

There are two channels of analogue signals for music recorded in stereo. The human ear is not sensitive to frequencies above 20 kHz, so each channel 

The small size of the pits, compared to the 0.78 pm wavelength of light and 1 pm diameter of the focussed spot used to read the disc, means that the disc diffracts rather than reflects light. Explanations that the light beam is not reflected when it hits a pit, are over simplifications. The regularly spaced pits in the radial direction act as a diffraction grating, therefore colours are observed when white light is reflected from the disc surface. 

Signals, from four detectors that scan the two diffraction peaks at 18° on either side of the disc surface normal (Fig. 14.19), are combined to give the 

The pits of the CD, of equal reflectivity to the rest of the disc, form a phase object. Light reflected from a pit of depth h = 0.12 pm is advanced in phase by 

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