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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.
          Protective lead aprons.
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
   Radio wavelengths with
    wavelengths in
    centimeters.
   Microwave ovens in the
    US use microwaves with a
    wavelength of about 12
    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.
    Radio Waves
   Waves with the
    lowers frequencies
    are radio waves: AM,
    FM, short-wave, and
    television.
                AM vs. FM Radio
       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
                 Radio Waves

Short-wave radios
     Longer wavelength than radio
     Reflect even better than radio
     International broadcasts

Television
      TV antenna or dish receives waves and
      uses them to make electrical current
                     Radar
   Radar waves are at the
    upper limit of the radio
    wave frequency.
   Radar systems use reflected
    radio waves to determine
    the distance to and from
    objects.                      Air traffic controllers and police
                                  officers both use radar systems
        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
    made of glass or plastic.
   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
             Automobile headlights
               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
    waves enter your eyes,
    your brain interprets
    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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