Light Waves

1
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

2
Electromagnetic Waves

• Remember our field induction rules
• Changing magnetic/electric field induces
  electric/magnetic field

3
Electromagnetic Spectrum

• Remember, the lower the frequency the longer the
  wavelength and vice versa
• Wave speed of light is 300,000,000 m/s

4
Colors

• Lowest frequencies of visible light are red
• Highest frequencies are violet
• The order is red, orange, yellow, green, blue,
  violet

5
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

6
Light and Materials

• Electromagnetic Waves carry energy
• When light strikes a material, it vibrates the
  lightweight electrons

7
Light and Materials
8
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 amplitude
• It can smack into the electrons in neighboring
  atoms and vibrate them too

9
Light and Materials

• This cuts down on the intensity of the light
• If all the light is absorbed by shaking electrons
  we say the material is opaque
• If the light frequency doesnt come close to the
  natural frequencies of the electrons, they shake
  a bit, but just re-emit the energy
• Materials that cant hold the energy are
  transparent

10
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

11
Light and Materials
See why the light travels more slowly in a
transparent material than in a vacuum?
12
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
-
13
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!!!

14
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

15
Colors

• 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

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Sunlight
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Colors

• 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

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Colors

• 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 absorbed

19
Mixing Colors

• Our eyes and brains are very complicated color
  processing sensors and computers
• See the discussion on page 289 in the text
• 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 eyes respond
  to

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Visual Response
21
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.
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Visual Response
Red Blue Magenta Red Green Yellow Blue
Green Cyan
Magenta Green White Cyan Red White Yellow
Blue White
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Selective Scattering

• This is why the sky is blue!
• The higher frequencies are scattered more by the
  molecules in the atmosphere
• Reds and oranges just pass through, so the sky
  appears blue

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Selective Scattering
Sunsets are red!