# Waves

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```					Waves
Ch.7
Waves and Energy

   MECHANICAL WAVES require a
medium to travel through (gas, solid,
liquids)
   Energy is transferred from particle to
particle in the medium.
   The energy travels along but the
particles (medium) does not.
Types of Waves

   A TRANSVERSE WAVE moves the
medium up and down, but the wave
(energy) moves left and right. (ex. Light)

   A LONGITUDINAL WAVE
(compressional) moves the medium
left and right and the wave (energy) also
left and right. (ex. sound)
Which is a
Transverse
Wave?
Wave Parameters
Wavelength (l)  length or size of one oscillation
Amplitude (A)  strength of disturbance (intensity)
Frequency (f)  repetition
Properties of Waves

 WAVELENGTH

 FREQUENCY

 AMPLITUDE
Wavelength
   WAVELENGTH is the distance between
crests.
   Wavelength = Speed/Frequency
What is the Wave length?
   Measure from any identical two successive points

5 10   15   20 25 30 35   40
What is the Wave length?
   Measure from any identical two successive points

5 10    15   20 25 30 35   40

30 - 10 = 20
What is the Wave length?
   Measure from any identical two successive points

5 10   15   20 25 30 35   40

22.5 - 2.5 = 20

   There are 4 complete oscillations depicted here
   ONE WAVE = 1 COMPLETE OSCILLATION
Frequency
   The number of wave crests
(wavelengths) that pass a given point
in a given amount of time.
   Measured in Hertz (Hz).
   The higher the frequency of the wave
the more energy the wave has.
   Frequency = Speed/Wavelength
Frequency
   Frequency = number of WAVES passing a stationary point
per second (Hertz)
Frequency and Period
Frequency (f) = number of oscillations passing by
per second
Period (T) = length of time for one oscillation
T = 1/f          f = 1/T
If a source is oscillating with a period of 0.1
seconds, what is the frequency?
f = 1/(0.1) = 10 Hz
It will complete 10 oscillations in one second.
(10 Hz)
If a source oscillates every 5 seconds, its
period is 5 seconds, and then the frequency
is               f = 1/5 = 0.2 Hz.
Wave Properties
Waves are oscillations and they transport energy.

The energy of a wave is proportional to its
frequency.
Fast oscillation = high frequency = high energy
Slow oscillation = low frequency = low energy

The amplitude is a measure of the wave intensity.
SOUND: amplitude corresponds to loudness
LIGHT: amplitude corresponds to brightness
Speed
   The speed of a wave differs depending
on the medium it travels through and
the energy the wave has.

   Speed = Wavelength x Frequency
Wave Speed
Wave speed depends on the wavelength and
frequency.
wave speed v = l f

Which animal can hear a shorter wavelength?
Cats (70,000 Hertz) or Bats (120,000 Hertz)

l = v/f
Wave Speed
v=lf

Which animal can hear a shorter wavelength?
Cats (70,000 Hertz) or Bats (120,000 Hertz)

l = v/f

Higher frequency = shorter wavelength

Lower frequency = longer wavelength
Amplitude
   AMPLITUDE is the
maximum distance the
particles in the medium
carrying the wave
(energy) move from
their rest position.

   The higher the
amplitude of the wave
the more energy the
wave has.
Frequency: Wave Properties Review
Number of waves passing a fixed position per second
f (cycles/second, Hertz)

Wavelength:
l Size of wave (in the direction of propagation)

Amplitude:
Size of wave (perpendicular to direction of propagation)
Proportional to Intensity(Sound loudness, Light brightness)

Wave Speed:    v=lf

Frequency increases            Frequency decreases
Energy increases               Energy decreases
Wavelength decreases           Wavelength increases
Wave behavior:

Reflection - the bouncing back of a
wave.
1) Sound echoes

2) Light images in
mirrors
3) Law of reflection
i = r
Refraction - the bending of a
wave caused by a change in
speed as the wave moves
from one medium to another.
Diffraction - the
bending of a wave
around the edge of
an object.
1) Water waves
bending around
islands
2) Water waves
passing through a slit
3) Diffraction depends on the
size of the obstacle or opening
compared to the wavelength of the
wave.

Less occurs if wavelength is   More occurs if wavelength is
smaller than the object.       larger than the object.
4) AM radio waves are longer
and can diffract around large
buildings and mountains; FM
can’t.
Interference - two
or more waves
overlapping to form
a new wave.
Resonance - the ability of an
object to vibrate by absorbing
energy at its natural frequency.
Sonic Boom
v > vsound
This fighter jet has just
accelerated past the speed of
sound. The sudden
decompression of the air causes
water droplets to condense,
forming a cloud.
Doppler Effect
   Change in frequency of a wave due to relative
motion between source and observer.

   A sound wave frequency change is noticed as a
change in pitch.
ELECTROMAGNETIC WAVES
   ELECTROMAGNETIC WAVES do not need
a medium to travel through.
   Some examples of electromagnetic waves
include:
TYPES OF ELECTROMAGNETIC
WAVES
GAMMA RAYS
 Emitted from the nuclei
of atoms during
during high-speed
collisions with
particles.
 Ionizing
 Used in cancer
treatment and for
sterilization
Sources: Cobalt 60, the
inner core of the sun
X-RAYS
   Emitted when an electron
moves from certain
excited states back down
to its ground state, or
when an electron that is
moving very quickly is
suddenly stopped
   Two groups - long
wavelength (soft x-rays)
and shorter wavelength
(hard x-rays)
ray photography) and to
look at materials in
industry for defects
   Sources: emitted by
heavy atoms after
bombardment by an
electron
ULTRAVIOLET
   Above the color violet
   Three groups - UV A,
UV B, and UV C.
   “A” type: longest
wavelength; least
harmful
    UV B and UV C are
absorbed by DNA in
cells
   Used by the body to
produce vitamin D, to
kill bacteria on
objects, and for sun
tanning
   Sources: Ultra hot
objects 5000°C or
more
VISIBLE LIGHT
   White light:         Color    Wavelength interval   Frequency interval
combination of all
the colors           red      ~ 625 to 740 nm       ~ 480 to 405 THz
   Rainbow: example of
white light that has orange   ~ 590 to 625 nm       ~ 510 to 480 THz
been separated into
yellow   ~ 565 to 590 nm       ~ 530 to 510 THz
a continuous
spectrum of colors green      ~ 520 to 565 nm       ~ 580 to 530 THz
   The names of colors
are assigned in      cyan     ~ 500 to 520 nm       ~ 600 to 580 THz
order of their
wavelengths          blue     ~ 430 to 500 nm       ~ 700 to 600 THz
   Used for
communications       violet   ~ 380 to 430 nm       ~ 790 to 700 THz
(fiber optics)
   Sources: very hot
objects
INFRARED

   Thought of as heat but
is not always
   Far infrared energy is
heat energy.
   All objects that have
infrared waves
   Easily absorbed and re-
   Used in remote
controls, surveillance,
therapy of muscles
   Sources: Humans, the
sun
MICROWAVES

   1 mm-1 dm in length
   Absorbed by water
molecules – how
microwave ovens heat
food
   Used in
telecommunications
and power
transmission
   Sources: electric
circuits, many stars,
microwave ovens
   10 cm- 100,000+m in
length
   Only cosmic waves
the reach the
surface of the Earth
   Cause of noise
   Divided into smaller
frequency
dependent groups
called bands
   Used
for communications
   Sources: transmitte
rs and sparks from
motors
Waves Light Up the Universe!
Waves?

remote controls, garage door openers

atmospheric research
Internet lesson
   Think you know all about the
electromagnetic spectrum? Well take
a tour of the Electromagnetic
Spectrum to find out more cool
information.
Light
   What is LIGHT?
   WHERE DOES IT COME FROM?
   (interesting article)
http://www.worldnetdaily.com/news/article.asp?ARTIC
LE_ID=54131
Light: particle or wave?
Light particles are called photons
Light: Particle or wave?
   BOTH!!
   Light does not need a medium to travel
through.
What is Light?
   Light is a wave, or rather acts like a
wave.
   How do we know?
 Reflection
 Refraction
 Dispersion
 Diffraction
 Interference
 Polarization
What is Light
   Light is a special type of wave
   What we know as light or VISIBLE
LIGHT is actually a type of something
called ELECTROMAGNETIC
   So, what is electromagnetic
waves?
Electromagnetic Waves
   When something creates energy it also
emits radiation. Depending on the amount
of energy, the object will emit different types
   When we studied mechanical waves, they
were all transferred through a medium.
What medium is light transferred through?
   LIGHT DOES NOT NEED ONE!
Electromagnetic Waves

   Electromagnetic waves are special
in the fact that they do not need a
medium to propagate through.
   But what is creating the
disturbance? What is emitting this
energy?
   ELECTRONS
Electromagnetic Waves
   Electrons in materials are vibrated and
emit energy in the form of photons,
which propagate across the universe.
   Photons have no mass, but are pure
energy.
   Electromagnetic Waves are waves that
are made up of these “photons”.
   When these photons come in contact
with boundaries, E-M waves interact
like other waves would.
Electromagnetic Waves

   Electromagnetic waves are everywhere.
   Light is only a small part of them

 TVs                          Lasers

 Microwaves                   CD/DVD players

 Light (Visible/UV/InfraRed)  X-Rays
Electromagnetic Spectrum
Speed of E/M Waves

   Previously, we found that
 V=f*l
   We also said that the speed of a wave in a certain
medium is always constant.
   It has been found that the speed of E-M waves
and light is ---
 3 x 108 or 300,000,000 m/s
 671,000,000 mph
 186,000 miles per second
 We call this value “c”
c=f*l

   C is constant throughout the universe, as
long as light is in a vacuum.
   When it is in other materials, c can
change, but can never be larger than its
value in a vacuum.
   Since “c” is constant, all of E-M waves
will have a corresponding frequency to
go along with their wavelength.
c = f * l ~~~ f = c / l
   Lets find the corresponding frequency ranges for a few of the
groups of E-M waves.
Energy in E-M Waves

   Which waves have more energy, Radio
waves or gamma waves?
   The greater the frequency of an E-M
wave, the more crests pass a point in a
certain amount of time, therefore the
more photons pass that point.
   This means that more energy moves past
that point in a certain amount of time or
that the wave contains more energy.
Back to Light

   So, why can we only see a small
portion of these E-M waves?
Our Eyes
Visible Light
   We now know what we see is part of the
electromagnetic spectrum. We know that
the light waves enter our eye, and stimulate
parts of it that cause a electrical impulse to
be sent to the brain which creates this visual
image.
   But everything does not emit radiation. How
do we see those things? And why cant we
see a window?
Seeing things
   We know that when waves run into a boundary
they are partially transmitted and partially
reflected.
   Light behaves as a wave, so it to is reflected.
   Therefore, an object does not need to emit
photons itself to be seen, it just has to reflect light
back to our eyes where we can detect it.
   Objects that do not allow light to pass through
them are called opaque.
   Objects that allow light to pass through them are
considered transparent.
   Objects in between are called translucent.
Polarization
   Polarization is a phenomenon of light
that is used in sun-glasses and 3-D
movies.
   Play with the two polarizing filters for a
few minutes and note what is
happening and see if you can think of
any reasons for it.
Polarization Hint
   Light vibrates in all directions.
   A polarizing filter acts like a picket
fence. It only lets certain direction
vibrations pass through it.
   Therefore, if you pass light through
two of them you can completely block
the light from passing through.
   HOW?
Polarization
   Electric and magnetic fields which
make up wave have preferred
direction
   Can be horizontal, vertical, circular, or
elliptical
   Most radio emission is unpolarized
Polarization
y
Electromagnetic Wave                                    Electric Field
Wave
Magnetic Field

x

Vertical Polarization               Horizontal Polarization

y                                    y

E

x                                    x           E
z                                      z
Polarization
Color
   Different objects may emit different
wavelengths of E-M radiation, so we would
see that light as different colors.
   But why do we see colors in objects that
reflect light? If you shine a white light on my
clothes, and it gets reflected why doesn’t all
of my clothes appear white?
   When I shine white light through a colored
piece of plastic, why does it change color?
Color
   The light we see is know as visible or white
light – although it is not that simple.
   The light is not really white, the white we see
is a combination of all the colors of the
rainbow.
   Remember R-O-Y G. B-I-V from art class.
   When all of these light waves are combined
we see white light.
Color Reflection
   So if we see something as WHITE, that
means …
   It reflected back all the wavelengths of
light to our eyes
   If we see something as RED or BLUE
 It reflected only the RED or only the BLUE
wavelengths
 The others were absorbed.

   And if we see something as black?
   It did not reflect back any of the light.
Color Transmission
   Filters work in a similar way.
 Red filters only let RED light thru.
 Blue let only BLUE light thru.

   What do you think that UV sticker