I saw the light

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I saw the light!!

• Chapter 27
• Light

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I. Theories of light

• A. Ancient Greeks
• Light travels from our eyes and bounces back
• B. Newton and others
• light was a stream of particles that go from a
  light source into our eyes
• Proof light moves in straight lines, reflection
• Demo with ball, laser, flashlight

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I. Theories of light

• C. In 1800s Thomas Young showed light acting like
  a wave
• Spreads out like a wave
• Light shows interference behavior

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• D. Through 1800s there was a debate between
  whether light was made of particles or waves
• E. Einstein
• light consists of massless bundles of
  electromagnetic energy called photons
• Photoelectric effect ex solar calculator

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• F. Light has a dual nature
• sometimes it acts like a particle
• sometimes it acts like a wave
• How does it act like a wave?
• How does it act like a particle?

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II. Speed of light

• A. Light is very very fast
• Too fast to be measured over short distances by
  everyday devices
• With sophisticated equipment it can be measured
  over long distances
• B. Represented by the letter c

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II. Speed of light

• C. Discovered by Michelson in 1880
• Used spinning mirrors and a mirror on a
  mountaintop
• First American to win the Nobel Prize
• 1907 for this discovery
• Michelson experiment

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• D. 299,920 km/s
• Round to 300,000 km/s
• 3 x 108 m/s
• 187,500 miles / sec
• Astronauts in space shuttle go 18,000 miles /
  hour or 5 miles/sec
• A beam of light could go around the earth 7.5
  times a sec!!

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Concept Quiz

• If a star 200 light years away from us blows up
  into a supernova today, when would we see it?
• 1. Tonight
• 2. 2 years
• 3. 200 years
• 4. not enough information

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Concept Quiz

• If something is one light year away, how many
  kilometers away is it?
• We will do this one together.

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• E. Remember v ?f
• Velocity stays at 3 x 108 m/s
• If frequency goes up wavelength goes down and
  vice versa

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III. Electromagnetic Waves

• A. Light energy emitted by accelerating
  electric charges
• electrons in atoms
• B. Travels in electromagnetic waves
• C. Visible light is a tiny part of the EM
  spectrum less than .1

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• electromagnetic spectrum wavelengths
• Stop here day one

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IV. Light and Transparent Material

• Brief atom review
• Made of protons, neutrons and electrons
• Always moving
• Sound analogy

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IV. Light and Transparent Material

• A. Light
• energy carried in an EM wave that is generated
  by vibrating electric charges
• B. When light shines on matter electrons in the
  matter are forced to vibrate.

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IV. Light and Transparent Material

• A. Light
• energy carried in an EM wave that is generated
  by vibrating electric charges
• B. When light shines on matter electrons in the
  matter are forced to vibrate.
• Vibrations are transferred

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• C. The natural vibration of electrons depends on
  how tightly they are held to the nucleus
• Visualize the electrons in an atom as connected
  by imaginary springs

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• C. The natural vibration of electrons depends on
  how tightly they are held to the nucleus
• Visualize the electrons in an atom as connected
  by imaginary springs
• Different materials have different spring
  strength and respond more to outside vibrations
  at some frequencies than others

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• D. How material responds depends on
• 1. the frequency of light
• 2. natural frequency of electrons in material

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• E. Glass
• 1. natural frequency in the ultra violet
• 2. When UV light shines on glass resonance occurs
• a. wave builds and get natural vibrations

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• 3. atom starts to vibrate more and more hitting
  other atoms and giving energy to those atoms in
  the form of heat

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• 3. atom starts to vibrate more and more hitting
  other atoms and giving energy to those atoms in
  the form of heat
• 4. glass stops UV rays
• 5. Instead of the energy going through the glass
  it gets turned into heat

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• F. When lower frequency light shines on glass
• 1. electrons forced to vibrate at smaller
  amplitudes

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• F. When lower frequency light shines on glass
• 1. electrons forced to vibrate at smaller
  amplitudes
• 2. holds energy for less time
• ?fewer collisions with neighbors
• ? less energy transferred as heat

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• 3. Energy of vibrating electrons is reemitted as
  light
• a. all visible light goes through glass
• b. frequency of light hitting it is same as
  frequency of light leaving it
• Color is the same leaving the glass as entering it

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• 4. Same frequency in as out but there is a time
  delay
• Speed of light slower in the glass

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V. Opaque Material

• A. Most materials absorb light without reemission
  of it
• No light passes through opaque
• B. Light hits it but vibrations are turned into
  random kinetic energy warming thing up

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• C. When light hits a material it can do three
  things
• 1. bounce off (reflect)
• 2. be transmitted through the material
• 3. be absorbed
• Usually a combination of all three

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VI. Shadows

• A. Ray thin beam of light
• A beam of light is a bundle of rays
• B. Shadow is formed where light rays cant reach
• When light shines on an object some of the rays
  may be stopped

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• C. Umbra total shadow
• D. Penumbra partial shadow
• Some of the light blocked, but other light fills
  in.
• E. Eclipse examples

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VII. Polarization

• A. Polarized vibrations go back and forth in
  one direction
• Up and down vertical polarization
• Side to side horizontal polarization

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• B. electrons emit EM waves that are polarized
• C. light sources have vibrating electrons in
  random directions so the light emitted is not
  polarized

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• D. When light shines on polarized filters the
  light gets polarized
• It blocks the light that is not in the same
  direction

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• E. Light reflects off horizontal surfaces
  horizontally
• Therefore sunglasses have a vertical axis
• Skipping stone analogy
• F. Used for 3D movies