Waves

							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
and spreading out
     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
  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
                     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
                   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
    warmth radiate
    infrared waves
   Easily absorbed and re-
    radiated.
   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
                   RADIO WAVES
   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!
    Why Do We Care About Radio
             Waves?

   Gadgets- cell phones, microwaves,
    remote controls, garage door openers

   Science- radio astronomy,
    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
    RADIATION.
   So, what is electromagnetic
    radiation and electromagnetic
    waves?
      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!
      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
        Radios                       Radiation

        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
   To learn more click here
                               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
    means on your sunglasses?
   Why do they sell those orange glasses
    that are supposed to reduce glare?