fit more than 783 megabytes (MB) onto a disc only 4.8
inches (12 cm) in diameter requires that the individual
bytes be very small. By examining the physical construction
of a CD, you can begin to understand just how small
these bytes are.
CD is a fairly simple piece of plastic, about four one-hundredths
(4/100) of an inch (1.2 mm) thick. Most of a CD consists
of an injection-molded piece of clear polycarbonate
plastic. During manufacturing, this plastic is impressed
with microscopic bumps arranged as a single, continuous,
extremely long spiral track of data. Once the clear
piece of polycarbonate is formed, a thin, reflective
aluminum layer is sputtered onto the disc, covering
the bumps. Then a thin acrylic layer is sprayed over
the aluminum to protect it. The label is then printed
onto the acrylic.
CD has a single spiral track of data, circling from
the inside of the disc to the outside. The fact that
the spiral track starts at the center means that the
CD can be smaller than 4.8 inches (12 cm) if desired,
and in fact there are now plastic baseball cards and
business cards that you can put in a CD player. CD business
cards hold about 2 MB of data before the size and shape
of the card cuts off the spiral.
data track is -- it is approximately 0.5 microns wide,
with 1.6 microns separating one track from the next.
(A micron is a millionth of a meter.) And the elongated
bumps that make up the track are each 0.5 microns wide,
a minimum of 0.83 microns long and 125 nanometers high.
(A nanometer is a billionth of a meter.)
will often read about "pits" on a CD instead
of bumps. They appear as pits on the aluminum side,
but on the side the laser reads from, they are bumps.
incredibly small dimensions of the bumps make the spiral
track on a CD extremely long. If you could lift the
data track off a CD and stretch it out into a straight
line, it would be 0.5 microns wide and almost 3.5 miles
(5 km) long!
read something this small you need an incredibly precise