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Chapter Eight:
Light
The Compact Disc (CD)
A
compact disc (CD) is a laser-read (also
called optically read) data storage device
on which music, video, or any type of
computer data can be stored. Two types in
popular use today are the CD audio and
video discs used for recording music and
television and the CD-ROM used to store any
type of computer data. CD-ROM stands for
Compact Disc Read-Only Memory. A CDROM can be read by a computer, and a CD
audio disc can be played by a compact audio
disc player. Both the CD-ROM and the CD
audio discs are made the same way and are
identical in structure. There are slight differences in the way CD audio drives and CDROM drives retrieve information, since
music is continuous and computer data is
stored in small, discrete chunks. Both drives,
however, have an optical sensor head with a
tiny diode laser, detection optics, and a
means of focusing. Both rotate between 200
to 500 revolutions per minute, compared to
the constant rotation of 331⁄3 revolutions per
minute for the old vinyl records. The CD
drive changes speed to move the head at a
constant linear velocity over the recording
track, faster near the inner hub and slower
near the outer edge of the disc. Furthermore,
the CD drive reads from the inside out, so the
disc will slow as it is played.
The CD itself is a 12 cm diameter, 1.3
mm thick sandwich of a hard plastic core, a
mirrorlike layer of metallic aluminum, and a
tough, clear plastic overcoating that protects
the thin layer of aluminum (box figure 8.8).
Technically, the disc does not contain any
operational memory, and the “Read-Only
Memory” is not actually memory either. It is
digitized data; music, video, or computer
data that have been converted into a string of
binary numbers. (See the reading on data
storage in chapter 7.) First, a master disc is
made. The binary numbers are translated
into a series of pulses that are fed to a laser.
The laser is focused onto a photosensitive
material on a spinning master disc. Whenever there is a pulse in the signal, the laser
burns a small oval pit into the surface, making a pattern of pits and bumps on the track
of the master disc. The laser beam is incredibly small, making marks about a micron or
so in diameter. A micron is one-millionth of
distinction between a particle and a wave. Evidence about this
strange nature of an extremely small-scale phenomenon will be
considered again in the next chapter as a basis for introducing the
quantum theory of matter.
SUMMARY
Electromagnetic radiation is emitted from all matter with a temperature
above absolute zero, and as the temperature increases, more radiation
and shorter wavelengths are emitted. Visible light is emitted from matter
hotter than about 700°C, and this matter is said to be incandescent. The
sun, a fire, and the ordinary lightbulb are incandescent sources of light.
The behavior of light is shown by a light ray model that uses straight
lines to show the straight-line path of light. Light that interacts with matter is reflected with parallel rays, moves in random directions by diffuse
reflection from points, or is absorbed, resulting in a temperature increase.
Matter is opaque, reflecting light, or transparent, transmitting light.
In reflection, the incoming light, or incident ray, has the same
angle as the reflected ray when measured from a perpendicular from the
point of reflection, called the normal. That the two angles are equal is
BOX FIGURE 8.8
This plastic disc with a 12 cm diameter
can store about 600 megabytes of data or
other information, the equivalent of about
300,000 typewritten pages.
—Continued top of next page
called the law of reflection. The law of reflection explains how a flat mirror forms a virtual image, one from which light rays do not originate.
Light rays do originate from the other kind of image, a real image.
Light rays are bent, or refracted, at the boundary when passing
from one transparent media to another. The amount of refraction
depends on the incident angle and the index of refraction, a ratio of the
speed of light in a vacuum to the speed of light in the media. When the
refracted angle is 90°, total internal reflection takes place. This limit to
the angle of incidence is called the critical angle, and all light rays with
an incident angle at or beyond this angle are reflected internally.
Each color of light has a range of wavelengths that forms the spectrum from red to violet. A glass prism has the property of dispersion,
separating a beam of white light into a spectrum. Dispersion occurs
because the index of refraction is different for each range of colors, with
short wavelengths refracted more than larger ones.
A wave model of light can be used to explain diffraction, interference, and polarization, all of which provide strong evidence for the
wavelike nature of light. Diffraction is the bending of light around the
edge of an object or the spreading of light in an arc after passing through
a tiny opening. Interference occurs when light passes through two small
slits or holes and produces an interference pattern of bright lines and
dark zones. Polarized light vibrates in one direction only, in a plane. Light
Chapter Eight:
Light
203
Continued—
a meter, so you can fit a tremendous number of data tracks onto the disc, which has
each track spaced 1.6 microns apart. Next,
the CD audio or CD-ROM discs are made
by using the master disc as a mold. Soft plastic is pressed against the master disc in a
vacuum-forming machine so the small
physical marks—the pits and bumps made
by the laser—are pressed into the plastic.
This makes a record of the strings of binary
numbers that were etched into the master
disc by the strong but tiny laser beam. During playback, a low-powered laser beam is
reflected off the track to read the binary
marks on it. The optical sensor head contains a tiny diode laser, a lens, mirrors, and
tracking devices that can move the head in
three directions. The head moves side to
side to keep the head over a single track
(within 1.6 micron), it moves up and down
to keep the laser beam in focus, and it moves
forward and backward as a fine adjustment
to maintain a constant linear velocity.
The advantages of the CD audio and
video discs over conventional records or tapes
include more uniform and accurate frequency response, a complete absence of background noise, and absence of wear—since
nothing mechanical touches the surface of the
disc when it is played. The advantages of the
CD-ROM over the traditional magnetic
floppy disks or magnetic hard disks include
storage capacity, long-term reliability, and the
impossibility of a head crash with a resulting
loss of data. Each CD-ROM, because of the
incredibly tiny size of the recording track, can
store about 600 megabytes (millions of bytes)
of data. This means that one 12 cm CD-ROM
disc will hold the same amount of data as 429
high-density, 1.4 megabyte, 3.5-inch floppy
disks (or 750 of the common double-sided,
800 kilobyte floppy disks).
The disadvantage of the CD audio and
CD-ROM discs is the lack of ability to do
writing or rewriting. Rewritable optical
media are available, for example, using the
magneto-optical method (M-O), which
combines magnetic recording with optical
techniques. M-O uses a plastic disc that has
a layer of magnetic particles embedded in
the plastic. To record digitized data a laser
heats a section of the track. At the same time
an electromagnet magnetizes the metallic
particle layer: north end up for a binary 1,
and south end up for a binary 0. The plastic
cools and “freezes” the binary information
can be polarized by certain materials, by reflection, or by scattering.
Polarization can only be explained by a transverse wave model.
A wave model fails to explain observations of light behaviors in
the photoelectric effect and blackbody radiation. Max Planck found
that he could modify the wave theory to explain blackbody radiation
by assuming that vibrating molecules could only have fixed amounts,
or quanta, of energy and found that the quantized energy is related to
the frequency and a constant known today as Planck’s constant. Albert
Einstein applied Planck’s quantum concept to the photoelectric effect
and described a light wave in terms of quanta of energy called photons. Each photon has an energy that is related to the frequency and
Planck’s constant.
Today, the properties of light are explained by a model that incorporates both the wave and the particle nature of light. Light is considered to have both wave and particle properties and is not describable in
terms of anything known in the everyday-sized world.
in the magnetic particles of the track. The
disc is read as a linearly polarized laser beam
is rotated clockwise or counterclockwise
according to the magnetic orientation of the
layer it is focused upon. The light is reflected
to a photodetector, where changes in light
intensity are interpreted as binary data. This
process can be used repeatedly as necessary.
Newer optical data storage technologies include a Compact Disc-Write-Once
method (CD-WO). This method uses a dyebased optical medium that absorbs heat
from the writing laser, changing color and
reflecting light differently for the reading
laser. Another common new technology is
the Write-Once Read-Many (WORM) system that writes data to a disc only once; the
data is then permanently stored. Currently,
WORM systems are used for records management and office functions that require a
large storage capacity and relatively fast
access. In the future, you may see systems
that store tremendous amounts of data on a
simple paper card the size of a credit card.
Could you imagine all of your textbooks
stored on cards that you could carry around
in one pocket? Perhaps you will read them
on a small, pocket-sized TV.
8.2
index of refraction n
speed of light in vacuum
speed of light in material
c
v
8.3
speed of light in vacuum (wavelength)(frequency)
c f
8.4
quantized
energy
( )
positive
whole
number
(
Planck’s
constant
)
(frequency)
E nhf
Summary of Equations
8.5
8.1
angle of incidence angle of reflection
i r
energy of
photon
(
E hf
Planck’s
constant
)
(frequency)