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A Closer Look

The Compact Disc (CD)

compact disc (CD) is a laser-read as a mold. Soft plastic is pressed against the coating used. Some of these dye and reflec-
A (also called optically read) data stor- master disc in a vacuum-forming machine tive coating combinations appear green,
age device. There are a number of different so the small physical marks—the pits and others appear blue, and still others appear
formats in use today, including music CDs, bumps made by the laser—are pressed into to be gold. Once a CD-R disc is burned, it
DVD movies, Blu-Ray DVD, and CDs for the plastic. This makes a record of the strings cannot be rewritten or changed.
storing computer data. All of these utilize of binary numbers that were etched into the The CD-RW is designed to have the
the general working principles described master disc by the strong but tiny laser beam. ability to do writing or rewriting. It uses a
below, but some have different refinements. During playback, a low-powered laser beam different technology but again mimics the
Some, for example, can fit much more data is reflected off the track to read the binary way light reflects from the pits and bumps
on a disc by utilizing smaller recording marks on it. The optical sensor head con- of a pressed commercial CD. Instead of a
tracks. tains a tiny diode laser, a lens, mirrors, and dye-based recording layer, the CD-RW uses
The CD disc rotates between 200 and tracking devices that can move the head in a compound made from silver, indium, anti-
500 revolutions per minute, but the drive three directions. The head moves side to side mony, and tellurium. This layer has a prop-
changes speed to move the head at a con- to keep the head over a single track (within erty that permits rewriting the information
stant linear velocity over the recording 1.6 micron), it moves up and down to keep on a disc. The nature of this property is that
track, faster near the inner hub and slower the laser beam in focus, and it moves forward when it is heated to a certain temperature
near the outer edge of the disc. Further- and backward as a fine adjustment to main- and cooled, it becomes crystalline. How-
more, the drive reads from the inside out, tain a constant linear velocity. ever, when it is heated to a higher tempera-
so the disc will slow as it is played. The disadvantage of the commercial ture and cooled, it becomes noncrystalline.
The CD disc is a 12 cm diameter, 1.3 mm CD is the lack of ability to do writing or A crystalline surface reflects a laser beam
thick sandwich of a hard plastic core, a mir- rewriting. Writing and rewritable optical while a noncrystalline surface absorbs the
rorlike layer of metallic aluminum, and a media are available, and these are called laser beam. The CD-RW is again designed
tough, clear plastic overcoating that protects CD-R and CD-RW. to mimic the way light reflects from the pits
the thin layer of aluminum. The CD records A CD-R records data to a disc by using and bumps of a commercial CD, except this
digitized data—music, video, or computer a laser to burn spots into an organic dye. time the string of binary numbers com-
data that have been converted into a string of Such a “burned” spot reflects less light than prises noncrystalline (nonreflective) and
binary numbers. First, a master disc is made. an area that was not heated by the laser. This crystalline areas (reflective). In order to
The binary numbers are translated into a is designed to mimic the way light reflects write, erase, and read, the CD-RW recorder
series of pulses that are fed to a laser. The laser from pits and bumps of a commercial CD, must have three different laser powers. It
is focused onto a photosensitive material on except this time the string of binary num- must have (1) a high power to heat spots to
a spinning master disc. Whenever there is bers comprises burned (nonreflective) and about 600°C, which cool rapidly and make
a pulse in the signal, the laser burns a small not burned areas (reflective). Since this is noncrystalline spots that are less reflective.
oval pit into the surface, making a pattern similar to how data on a commercial CD are It must have (2) a medium power to erase
of pits and bumps on the track of the mas- represented, a CD-R disc can generally be data by heating the media to about 200°C,
ter disc. The laser beam is incredibly small, used in a CD player as if it were a commer- which allows the media to crystallize and
making marks about a micron or so in diam- cial CD. The dyes in a CD-R disc are photo- have a uniform reflectivity. Finally, it must
eter. A micron is one-millionth of a meter, sensitive organic compounds that are simi- have (3) a low setting that is used for find-
so you can fit a tremendous number of data lar to those used in making photographs. ing and reading nonreflective and the more
tracks onto the disc, which has each track The color of a CD-R disc is a result of the reflective areas of a disc. The writing and
spaced 1.6 microns apart. Next, commercial kind of dye that was used in the recording rewriting of a CD-RW can be repeated hun-
CD discs are made by using the master disc layer combined with the type of reflective dreds of times.

a book at a certain height above a lake (Figure 7.24). You can might say that this behavior is not only strange but impossible.
make measurements and calculate the kinetic energy the book Yet it is an analogy to the observed behavior of light.
will have when dropped into the lake. When it hits the water, the As stated, light has a dual nature, sometimes exhibiting the
book disappears, and water waves move away from the point of properties of a wave and sometimes exhibiting the properties
impact in a circular pattern that moves across the water. When of moving particles but never exhibiting both properties at the
the waves reach another person across the lake, a book identical same time. Both the wave and the particle nature are accepted
to the one you dropped pops up out of the water as the waves as being part of one model today, with the understanding that
disappear. As it leaves the water across the lake, the book has the the exact nature of light is not describable in terms of any-
same kinetic energy that your book had when it hit the water thing that is known to exist in the everyday-sized world. Light
in front of you. You and the other person could measure things is an extremely small-scale phenomenon that must be diff er-
about either book, and you could measure things about the ent, without a sharp distinction between a particle and a wave.
waves, but you could not measure both at the same time. You Evidence about this strange nature of an extremely small-scale

7-19 CHAPTER 7 Light 195