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# Light

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```									    Light
Part 1 – Electromagnetic
Spectrum
Modeling Light
   Light can be described using two different
models: the wave model or the particle
model.
Wave Model
   The wave model describes
light as transverse waves
that do not require a
medium to travel through.

 Describe a transverse wave.
 What type of wave does not
need a medium to travel?
Particle Model
   Discussion: Observations that could not be
explained by the wave model…

   The particle model assumes that light is
contained in small packets called photons.

   Photons do not have mass, they are more
like bundles of energy.
Red Light vs. Blue Light
Wave Model & Particle Model
   According to the
wave model, red
light should have
more energy
because it has a
greater
amplitude… so
why couldn’t it
knock electrons
off the metal
plate?
The Speed of Light
   During a thunder storm, what would
happen first: you hear the thunder or you
see the bolt of lightning?

   In a vacuum, all light travels at the same
speed called c : 3 x 108 meters/second
   Light is the fastest signal in the universe

** You should memorize this number!! All electromagnetic waves travel at this speed.
Speed of Light
   The speed of light depends on the
medium that it travels through.
   What type of medium is in a vacuum?

   Light travels slower
through slower through
a medium than it does
in a vacuum.
Intensity
   The quantity that measures how bright a light is,
or the amount of light that illuminates a surface,
is called intensity.

   On which squares is the light most intense?
The electromagnetic spectrum (EM spectrum) consists of
light at all possible energies, frequencies, and wavelengths. It
includes radio waves, ultraviolet waves, visible waves, x-rays,
and more.
Our eyes can
detect light waves
ranging from
400nm (violet
light) to 700nm
(red light). This is
called the visible
light spectrum.

Remember:
ROYGBIV
Sunlight
   Sunlight contains ultraviolet light (UV
light)
   UV light has higher energy and shorter
wavelengths than visible light.
   UV light causes sunburns. Prolonged or
repeated exposure may lead to skin
cancer.
UV Rays are good too…
   Cause skin cells to manufacture vitamin D
for healthy bones and teeth.
   Used in hospitals for cleanliness
   Kill bacteria
Class Work
   Real World Applications p.403
X – Rays

   High frequency
electromagnetic
waves, beyond
the UV part of the
EM spectrum.
   Used in taking pictures
of the inside of the
body.
   X – ray Passes through
low density objects but is
absorbed by high density
objects, like bones and
teeth.
   X rays can be harmful to
humans because they
can kill healthy body
cells, or turn them into
cancer cells.
Infrared (IR) Light

   Wavelength longer
than visible light.
   IR light from the sun
or a lamp warms you.
   IR light keeps
cafeteria food warm.
   Given off by hot
objects as they cool.
   Thermogram - A special IR film
that detects the amount of IR
radiation given off. It can identify
warm or cold areas of an object
or person. Large amounts = Red,
Small amounts = Blue.

   IR sensitive binoculars enable
people to “see” objects in fog or
complete darkness.
Microwaves
wavelengths in
centimeters.
   Microwave ovens in the
US use microwaves with a
cm.
   Microwaves are reflected
by metals, and go through
paper and plastic, but are
absorbed by water, fat,
and sugars.
   Microwaves are also used to carry
telecommunication signals.

   Microwaves will create large currents of
electricity and can be harmful with people
wearing heart pacemakers.
   Waves with the
lowers frequencies
FM, short-wave, and
television.
AM                              FM
Vary amplitude                Amplitude constant

Frequency constant:           Frequency Varies:
535 kilohertz (535,000) -    88 megahertz (88,000,000) -
1605 kilohertz (1,605,000)    107.9 megahertz (107,900,000)

Good over long distances

AM travels longer distances than FM waves
because AM has a longer wavelength
Waves

Television
TV antenna or dish receives waves and
uses them to make electrical current
   Radar waves are at the
wave frequency.
the distance to and from
objects.                      Air traffic controllers and police
Light
Part 2 – Reflection, Refraction,
Lenses & Color
Electromagnetic Waves

   All types of electromagnetic waves have
the same properties as visible light.
   They can be reflected and refracted.
 Reflected: Bounces off an object.

 Refracted: Bends going from one
material to another.
Another Light Model

   A light ray is a model of light that
represents light traveling through space in
a straight line.
   Every object reflects some light and
absorbs some light.
   Light rays can be drawn to show how light
is reflected off a surface.
Rough surfaces reflect light in many
directions… sometimes called a diffuse
reflection
Reflection: The return of light or sound waves
from a substance

Angle of Incidence = Angle of reflection
Refraction
The bending of light waves as they pass from
one material into another

i

r

Θi = Θi
Θr = Θr                r

i

As light enters a denser material it bends toward the normal
Index of Refraction
The ratio between the speed of light in a vacuum and the
speed of light in a particular substance

Hydrogen                    1.00013
Air                        1.00029
Water                      1.33
Glass                      1.5 to 2.0
Diamond                    2.42

** The higher the index of refraction, the more the light will slow and bend.
Lenses

   A lens is a transparent object, usually
   The lens is curved so that it changes that
direction of light.
   The amount the lens is curved and the
lens material determines how much the
light will be refracted.
Convex Lens
A lens that is thicker at the center than at the edges

Glasses for people that are far sighted
Telescopes
Microscopes
Concave Lens
A lens thinner at its center than at the edges

Glasses for people who are nearsighted
Peepholes for doors
Some wide angle camera lenses
Concave mirror
A spherical reflecting surface that constitutes a
segment of the interior of a sphere

Flashlights
Convex mirror
A spherical reflecting surface that constitutes a
segment of the exterior of a sphere.

Makes objects appear to be further away because objects
look smaller when you look at them in your mirror
Seeing Colors
   Different wavelengths of visible light correspond
to different colors.

   Your brain interprets each wavelength of visible
light as a certain color.
   Every object absorbs or
reflects visible light
waves.
   The reflected light
them as colors.

White light is the combination of all
visible light.
   A prism is a transparent block that can
separate white light into its component
colors, ROYGBIV.
   Water in the air can act as a prism,
separating white sunlight into the colors of
the rainbow.
Waves

Laser
A device that produces a beam of highly concentrated
light that is all one frequency or color
Light, flashlight, or the sun:
The light from these sources is usually a mixture
of colors at different frequencies and
wavelengths
Light waves are all jumbled and independent of
each other once they leave the source
They go everywhere and become scattered
Waves

Lasers

Waves are all the same frequency and wavelength

Crests and troughs line up and amplitudes are the same

All waves move in the same direction from the source
Waves
Definition of: laser

(Light Amplification by the Stimulated Emission of Radiation)
A device that creates a uniform and coherent light that is very
different from an ordinary light bulb. Many lasers deliver light
in an almost-perfectly parallel beam (collimated) that is very
pure, approaching a single wavelength. Laser light can be
focused down to a tiny spot as small as a single wavelength.

Laser output can be continuous or pulsed and is used in a
myriad of applications. Gas lasers are used to cut steel and
perform delicate eye surgery, while solid state lasers create
the ultra-high-speed, miniscule pulses traveling in optical
fibers traversing the backbones of all major communications
networks. Light traveling in an optical fiber is impervious to
external interference, a constant problem with electrical
pulses in copper wire.
Waves
LASER
How Does It Work?

• A laser is an optical oscillator, which is made out of a solid, liquid or gas
with mirrors at both ends. To make the laser work, the material is excited
or "pumped," with light or electricity. The pumping excites the electrons in
the atoms, causing them to jump to higher orbits, creating a "population
inversion."
• A few of the electrons drop back to lower energy levels spontaneously,
releasing a photon (quantum of light). The photons stimulate other
excited electrons to emit more photons with the same energy and thus
the same wavelength as the original.
• The light waves build in strength as they pass through the laser
medium, and the mirrors at both ends keep reflecting the light back and
forth creating a chain reaction and causing the laser to "lase.“
• In simple laser cavities, one mirror has a small transparent area that
lets the laser beam out. In semiconductor lasers, both mirrors often
transmit a beam, the second one being used for monitoring purposes.
Waves
Unit # 7 : Waves: Pretest Problems

All electromagnetic waves travel at ________ meters/second

The speed of sound @ 0 o C. is _________ meters/second

The speed of sound @ 30 o C. is _________ meters/second

The change in the speed of sound per change of 1 oC. is ____

The speed of sound @ 50 o C. is _______ meters/second .

The speed of sound is 380 meters/second .
The temperature is _______   o   C.

In the summer time (speed of sound : 350 meters/second ) ,
how far could a sound wave travel in a day ?
Waves
Unit # 7 : Waves: Pretest Problems

On a cold winter day (0 o C.) you beep your snowmobile
horn .The sound bounces off a cliff and returns to
you in 4 seconds . How far away is the cliff ?

Three people are playing jump rope . One person shakes
the rope 12 times in 3 seconds . The speed of the
rope is 20 meters per second . How long is the
wavelength ?

At 30 o C. a tuning fork has a wavelength of 5 meters .
What is the frequency of the sound it produces ?

The speed of a wave is 3.0 x 10 8 meters/second . The
wavelength is 3 meters . What is the frequency
of the wavelength ?

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