Light Waves - PowerPoint

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					            Light Waves
• What we call light is a small portion of
  the electromagnetic spectrum
• All the different colors are
  electromagnetic waves with different
  wave lengths
• Wave speed = frequency x wavelength
• EM radiation is alternating electric and
  magnetic fields
    Electromagnetic Waves
• Remember our field induction rules
• Changing magnetic/electric field
  induces electric/magnetic field
  Electromagnetic Spectrum
• Remember, the lower the frequency the
  longer the wavelength and vice versa
• Wave speed of light is 300,000,000 m/s
• Lowest frequencies of visible light are
• Highest frequencies are violet
• The order is red, orange, yellow, green,
  blue, violet
      Producing EM Waves
• Need to vibrate charged particles
• We shake electrons in an antenna to
  make radio and TV waves
• We shake electrons in a magnetron to
  make microwaves
• We shake electrons in atoms to make
  light (this is a bit of a simplification
  which we will correct later this semester
        Light and Materials
• Electromagnetic Waves carry energy
• When light strikes a material, it vibrates
  the lightweight electrons
Light and Materials
        Light and Materials
• If the natural vibration frequency of the
  electrons is close to the frequency of
  the incoming light wave, the electron will
  be set into big vibrations (resonance)
• It will absorb some energy from the EM
  wave and will vibrate with larger
• It can smack into the electrons in
  neighboring atoms and vibrate them too
        Light and Materials
• This cuts down on the intensity of the
• If all the light is absorbed by shaking
  electrons we say the material is opaque
• If the light frequency doesn’t come close
  to the natural frequencies of the
  electrons, they shake a bit, but just re-
  emit the energy
• Materials that can’t hold the energy are
        Light and Materials
• There is a delay in transparent materials
  between the time the electrons start
  shaking and then re-emit the energy
• This means that the light travels more
  slowly in these materials!!!
• We will use this fact later to do
  interesting things with lenses
   Light and Materials

See why the light travels more
slowly in a transparent material than
in a vacuum?
       Light and Materials
• Infrared EM waves shake entire atoms
  or molecules in materials
• Microwave EM waves effect the rotation
  of atoms about each other
            Remember our discussion of
            water and how it has a
            positive end by the hydrogen
            atoms and a negative end by
            the oxygen
        Light and Materials
• Things that are opaque absorb the
  energy carried by the light wave
• This means that they heat up
• Metals are opaque and shiny
• The free electrons close to the surface
  shake and re-emit the energy back out
  of the material
• They are reflectors!!!
            Light and Air
• Our atmosphere is transparent to visible
  light, some ultraviolet light and some
  infrared light
• The ozone layer absorbs a lot of
  ultraviolet light
• The ozone molecule is O3 and breaks
  into a regular oxygen molecule O2 and
  an oxygen atom
• Colored objects are a result of selective
  reflection and absorption of light
• Sunlight is a mixture of colors
• We call this mixture white light
• When white light strikes a red object,
  the red frequencies are reflected and
  the other frequencies are absorbed
• White objects reflect all frequencies
• Black objects absorb all frequencies
• Objects can only reflect the colors of
  light that shine on them
• If you shine blue light on a red object it
  will look black
• We can also consider shining light
  through semi-transparent objects
• Filters used on the lights in a theater are
  materials that absorb some frequencies
  and let other frequencies pass through
• Red filters transmit red light through the
  material while other frequencies are
           Mixing Colors
• Our eyes and brains are very
  complicated color processing sensors
  and computers
• See the discussion on page 289 in the
• Artists have learned how to mix colors
  to make pleasing combinations and to
  create a huge array of different colors
• All based on what the sensors in our
Visual Response
             Visual Response
Red + Blue = Magenta
Red + Green = Yellow
Blue + Green = Cyan

                        The way your
                        computer screen
                        makes colors. And
                        your TV set does the
                        same thing.
             Visual Response
Red + Blue = Magenta
Red + Green = Yellow
Blue + Green = Cyan

                       Magenta + Green =
                       Cyan + Red = White
                       Yellow + Blue = White
       Selective Scattering
• This is why the sky is blue!
• The higher frequencies are scattered
  more by the molecules in the
• Reds and oranges just pass through, so
  the sky appears blue
           Selective Scattering

are red!