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Concept Version 7
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Using Interference to Read CDs and DVDs

Optical discs are digital storing media read in an optical disc drive using laser beam.

Learning Objective

  • Explain how information is stored on the optical disks


Key Points

    • The iridescent layer of the disc is imprinted with tiny pits and lands. Pits scatter light when illuminated, and produce a reading of 0; lands reflect light back and produce a reading of 1.
    • The optical disc drive records the 0 and 1 readings and translates them into binary data which is used to relay whatever information is recorded on the disc.
    • Rainbow pattern on the back of a CD is due diffraction of the reflected light by pits.

Terms

  • land

    A flat area on an optical disc that reflects light when illuminated.

  • binary data

    Data which can take on only two possible values, traditionally termed 0 and 1.

  • pit

    An imprint on an optical disc that scatters light when illuminated.


Full Text

Overview

Compact disks (CDs) and digital video disks (DVDs) are examples of optical discs. They are read in an optical disc drive which directs a laser beam at the disc. The reader then detects whether the beam has been reflected or scattered.

Function of Digital Discs

Optical discs are digital storing media. They can store music, files, movies, pictures etc.. These discs are flat, usually made of aluminum, and have microscopic pits and lands on one of the flat surfaces (as shown in ). The information on these discs are read by a computer in the form of binary data. First, a laser beam is shot at the disc. If the beam hits a land, it gets reflected back and is recorded as a value of 1. If the beam hits a pit, it gets scattered and is recorded as a value of zero.

Early Version of an Optical Disc

In this early version of an optical disc, you can see the pits and lands which either reflect back light or scatter it.

These microscopic pits and lands cover the entire surface of the disc in a spiral path, starting in the center and working its way outward. The data is stored either by a stamping machine or laser and is read when the data is illuminated by a laser diode in the disc drive. The disc spins at a faster speed when it is being read in the center track, and slower for an outer track. This is because the center tracks are smaller in circumference and therefore can be read quicker.

These pits also act as slits and cause the light to be diffracted as it is reflected back, which causes an iridescent effect. This explains the rainbow pattern that you see on the back of a CD, as shown in .

Compact Disc

The bottom surface of a compact disc showing characteristic iridescence.

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