Activity Lab Exercise: Crossover Cable
ConstructionIn this lab, the student will build a Category 5 or
Category 5e (CAT 5 or 5e) unshielded twisted pair (UTP)
Ethernet crossover cable to T568-B and T-568-A standards and
test the cable for continuity and correct pin-outs, correct
wire on the right pin. Lab Activity Lab Exercise: UTP
Cable PurchaseThis lab will introduce the variety and prices of
network cabling and components in the market. The student will
gather pricing information for UTP patch cables and bulk
cable. Web Links Networking Basic's Cables and Stuff
http://www.waterwheel.com/Guides/ networking_basics_0006.htm
Content 3.2 Optical Media 3.2.1
The electromagnetic spectrum The light used in optical
fiber networks is one type of electromagnetic energy. When an
electric charge moves back and forth, or accelerates, a type of
energy called electromagnetic energy is produced. This energy
in the form of waves can travel through a vacuum, the air, and
through some materials like glass. An important property of any
energy wave is the wavelength. Radio, microwaves, radar,
visible light, x-rays, and gamma rays seem to be very different
things. However, they are all types of electromagnetic energy.
If all the types of electromagnetic waves are arranged in order
from the longest wavelength down to the shortest wavelength, a
continuum called the electromagnetic spectrum is created. The
wavelength of an electromagnetic wave is determined by how
frequently the electric charge that generates the wave moves
back and forth. If the charge moves back and forth slowly, the
wavelength it generates is a long wavelength. Visualize the
movement of the electric charge as like that of a stick in a
pool of water. If the stick is moved back and forth slowly, it
will generate ripples in the water with a long wavelength
between the tops of the ripples. If the stick is moved back and
forth more rapidly, the ripples will have a shorter wavelength.
Because electromagnetic waves are all generated in the same
way, they share many of the same properties. They all travel at
a rate of 300,000 kilometers per second (186,283 miles per
second) through a vacuum. Human eyes were designed to only
sense electromagnetic energy with wavelengths between 700
nanometers and 400 nanometers (nm). A nanometer is one
billionth of a meter (0.000000001 meter) in length.
Electromagnetic energy with wavelengths between 700 and 400 nm
is called visible light. The longer wavelengths of light that
are around 700 nm are seen as the color red. The shortest
wavelengths that are around 400 nm appear as the color violet.
This part of the electromagnetic spectrum is seen as the colors
in a rainbow. Wavelengths that are not visible to the human eye
are used to transmit data over optical fiber. These wavelengths
are slightly longer than red light and are called infrared
light. Infrared light is used in TV remote controls. The
wavelength of the light in optical fiber is either 850 nm, 1310
nm, or 1550 nm. These wavelengths were selected because they
travel through optical fiber better than other wavelengths.
Web Links Electromagnetic Spectrum
http://imagine.gsfc.nasa.gov/docs/science/
know_l1/emspectrum.html
Content 3.2 Optical
Media 3.2.2 Ray model of light When
electromagnetic waves travel out from a source, they travel in
straight lines. These straight lines pointing out from the
source are called rays. Think of light rays as narrow beams of
light like those produced by lasers. In the vacuum of empty
space, light travels continuously in a straight line at 300,000
kilometers per second. However, light travels at different,
slower speeds through other materials like air, water, and
glass. When a light ray called the incident ray, crosses the
boundary from one material to another, some of the light energy
in the ray will be reflected back. That is why you can see
yourself in window glass. The light that is reflected back is
called the reflected ray. The light energy in the incident ray
that is not reflected will enter the glass. The entering ray
will be bent at an angle from its original path. This ray is
called the refracted ray. How much the incident light ray is
bent depends on the angle at which the incident ray strikes the
surface of the glass and the different rates of speed at which
light travels through the two substances. The bending of light
rays at the boundary of two substances is the reason why light
rays are able to travel through an optical fiber even if the
fiber curves in a circle. The optical density of the glass
determines how much the rays of light in the glass bends.
Optical density refers to how much a light ray slows down when
it passes through a substance. The greater the optical density
of a material, the more it slows light down from its speed in a
vacuum. The ratio of the speed of light in a material to the
speed of light in a vacuum is called the Index of Refraction.
Therefore, the measure of the optical density of a material is
the index of refraction of that material. A material with a
large index of refraction is more optically dense and slows
down more light than a material with a smaller index of
refraction. For a substance like glass, the Index of
Refraction, or the optical density, can be made larger by
adding chemicals to the glass. Making the glass very pure can
make the index of refraction smaller. The next lessons will
provide further information about reflection and refraction,
and their relation to the design and function of optical fiber.
Interactive Media Activity Interactivity: Propagation
of Light in Matter This activity demonstrates how light slows
down through different materials. Web Links Reflection
and Mirrors http://www.glenbrook.k12.il.us/gbssci/
phys/Class/refln/r eflntoc.html
Content 3.2
Optical Media 3.2.3 Reflection When a ray
of light (the incident ray) strikes the shiny surface of a flat
piece of glass, some of the light energy in the ray is
reflected. The angle between the incident ray and a line
perpendicular to the surface of the glass at the point where
the incident ray strikes the glass is called the angle of
incidence. The perpendicular line is called the normal. It is
not a light ray but a tool to allow the measurement of angles.
The angle between the reflected ray and the normal is called
the angle of reflection. The Law of Reflection states that the
angle of reflection of a light ray is equal to the angle of
incidence. In other words, the angle at which a light ray
strikes a reflective surface determines the angle that the ray
will reflect off the surface. Interactive Media
Activity Interactivity: Law of Reflection This activity
animates how angles affect reflection. Web Links The
Transmission of Wave through Dense media – Reflection and
Refraction http://www.phy.ntnu.edu.tw/java/ propagation/
propagation.html
Content 3.2 Optical Media
3.2.4 Refraction When a light strikes the
interface between two transparent materials, the light divides
into two parts. Part of the light ray is reflected back into
the first substance, with the angle of reflection equaling the
angle of incidence. The remaining energy in the light ray
crosses the interface and enters into the second substance. If
the incident ray strikes the glass surface at an exact
90-degree angle, the ray goes straight into the glass. The ray
is not bent. However, if the incident ray is not at an exact
90-degree angle to the surface, then the transmitted ray that
enters the glass is bent. The bending of the entering ray is
called refraction. How much the ray is refracted depends on the
index of refraction of the two transparent materials. If the
light ray travels from a substance whose index of refraction is
smaller, into a substance where the index of refraction is
larger, the refracted ray is bent towards the normal. If the
light ray travels from a substance where the index of
refraction is larger into a substance where the index of
refraction is smaller, the refracted ray is bent away from the