Waves Ch'7

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WavesCh.7
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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.

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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)

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Which is a Transverse Wave?
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Wave Parameters

• Wavelength (l) ? length or size of one
  oscillation
• Amplitude (A) ? strength of disturbance
  (intensity)
• Frequency (f) ? repetition

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Properties of Waves

• WAVELENGTH
• FREQUENCY
• AMPLITUDE

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Wavelength

• WAVELENGTH is the distance between crests.
• Wavelength Speed/Frequency

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What is the Wave length?

• Measure from any identical two successive points

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What is the Wave length?

• Measure from any identical two successive points

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30 - 10 20
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What is the Wave length?

• Measure from any identical two successive points
• There are 4 complete oscillations depicted here
• ONE WAVE 1 COMPLETE OSCILLATION

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22.5 - 2.5 20
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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

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Frequency

• Frequency number of WAVES passing a stationary
  point per second (Hertz)

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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.

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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

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Speed

• The speed of a wave differs depending on the
  medium it travels through and the energy the wave
  has.
• Speed Wavelength x Frequency

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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

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Wave Speed

• v l f
• 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

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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.

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Wave Properties Review
Frequency 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 l
f Frequency increases Frequency
decreases Energy increases
Energy decreases Wavelength decreases
Wavelength increases
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Wave behavior
Reflection - the bouncing back of a wave.
1) Sound echoes
2) Light images in mirrors 3)
Law of reflection i r
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Refraction - the bending of a wave caused by a
change in speed as the wave moves from one medium
to another.
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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
and spreading out
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3) Diffraction depends on the size of
the obstacle or opening compared to the
wavelength of the wave.
Less occurs if wavelength is smaller than the
object.
More occurs if wavelength is larger than the
object.
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4) AM radio waves are longer and can
diffract around large buildings and mountains FM
cant.
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Interference - two or more waves overlapping to
form a new wave.
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Resonance - the ability of an object to vibrate
by absorbing energy at its natural frequency.
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Sonic Boom

• v gt 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.
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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.

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ELECTROMAGNETIC WAVES

• ELECTROMAGNETIC WAVES do not need a medium to
  travel through.
• Some examples of electromagnetic waves include

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TYPES OF ELECTROMAGNETIC WAVES

• GAMMA RAYS
• Emitted from the nuclei of atoms during
  radioactive decay or during high-speed collisions
  with particles.
• Ionizing
• Used in cancer treatment and for sterilization
  Sources Cobalt 60, the inner core of the sun

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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)
• Used for radiography (x-ray photography) and to
  look at materials in industry for defects
• Sources  emitted by heavy atoms after
  bombardment by an electron

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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 5000C or more

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VISIBLE LIGHT

• White light combination of all the colors
• Rainbow example of white light that has been
  separated into a continuous spectrum of colors
• The names of colors are assigned in order of
  their wavelengths
• Used for communications
• (fiber optics)
• Sources   very hot objects

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INFRARED

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

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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

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RADIO WAVES

• 10 cm- 100,000m  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  transmitters and sparks from motors

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Waves Light Up the Universe!
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Why Do We Care About Radio Waves?

• Gadgets- cell phones, microwaves, remote
  controls, garage door openers
• Science- radio astronomy, atmospheric research

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Internet lesson

• Think you know all about the electromagnetic
  spectrum? Well take a tour of the Electromagnetic
  Spectrum to find out more cool information.

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Light

• What is LIGHT?
• WHERE DOES IT COME FROM?
• (interesting article) http//www.worldnetdaily.com
  /news/article.asp?ARTICLE_ID54131

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Light particle or wave?
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Particle or wave?
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Light particles are called photons
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Light Particle or wave?

• BOTH!!
• Light does not need a medium to travel through.

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What is Light?

• Light is a wave, or rather acts like a wave.
• How do we know?
• Reflection
• Refraction
• Dispersion
• Diffraction
• Interference
• Polarization

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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 RADIATION.
• So, what is electromagnetic radiation and
  electromagnetic waves?

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Electromagnetic Waves

• When something creates energy it also emits
  radiation. Depending on the amount of energy,
  the object will emit different types of
  electromagnetic radiation.
• When we studied mechanical waves, they were all
  transferred through a medium. What medium is
  light transferred through?
• LIGHT DOES NOT NEED ONE!

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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

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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.

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Electromagnetic Waves

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

• Radiation
• Lasers
• CD/DVD players
• X-Rays

• Radios
• TVs
• Microwaves
• Light (Visible/UV/InfraRed)

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Electromagnetic Spectrum
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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

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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.

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c f l f c / l

• Lets find the corresponding frequency ranges for
  a few of the groups of E-M waves.

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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.

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Back to Light

• So, why can we only see a small portion of these
  E-M waves?

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Our Eyes
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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?

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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.

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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.

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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?

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Polarization

• Electric and magnetic fields which make up wave
  have preferred direction
• Can be horizontal, vertical, circular, or
  elliptical
• Most radio emission is unpolarized
• To learn more click here

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Polarization
Electric Field
Electromagnetic Wave
Magnetic Field
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Polarization
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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 doesnt all of my
  clothes appear white?
• When I shine white light through a colored piece
  of plastic, why does it change color?

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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.

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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.

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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 means on your
  sunglasses?
• Why do they sell those orange glasses that are
  supposed to reduce glare?