Light Waves

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Light Waves
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Speed of LightIn a Vacuum

• C
• 186,282.3959 mi/sec
• or
• 299,792,458 m/s
• We'll use 3x108 m/s for C in vacuum and in air.

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Speed of Light in a Medium
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If you keep pushing the log forward, where will
it end up?
Concrete
Mud
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Refraction
Air
Glass
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Concave Lens
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Convex Lens
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Diffraction

• All waves bend around corners, edges, and
  barriers.

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Diffraction

• All waves bend around corners, edges, and
  barriers.
• Sound waves go around corners.
• A wave passing through a narrow slit fans out.
• Looking through a tiny hole can substitute for
  glasses.

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Double Slit Interference
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Double Slit Demo
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Bright
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Dark
Bright
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Bright
Dark
Bright
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Double Slit Interference
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Side Note

• Done with a laser because the light has a single
  frequency and is in phase.
• This is not something you see every day because
  most light is mixed and fills in the dark spots.
• Before lasers, scientists used sodium lights.

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Limit of Diffraction

• Microwaves in your kitchen are about 12 cm long
  which is too big to fit through the the holes of
  the screen in your microwave. To waves that
  long, the screen looks solid and they are
  reflected back as if from a mirror.
• Note it is not the amplitude that determines if
  the wave is too large to fit through, but the
  wavelength.

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Anatomy of a Light Wave
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How a Microwave Heats Food

• Normal Microwave Use
• Polar Molecule has () end and (-) end a
  dipole. They are attracted to / repelled by the
  electric field.
• Field orients the molecule to match.
• Filed changes direction so dipole flips to
  reorient itself
• Repeat.
• Molecular motion heat

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Wait a second?

• Q All light is electromagnetic waves, so why
  doesn't all light heat things the way a microwave
  does?

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• A1 Resonance. Microwaves match the natural
  frequency of water so the molecule flips wildly,
  other waves just make the molecules jiggle a bit.
• A2 Microwaves are 800 to 1,000 Watts. Have you
  ever touched a 500 W bulb?
• A3 Microwaves can pass further into or through
  many substances because of their long wavelengths.

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Polarizing Filters

• Sunglasses and Computer Screen
• Sunglasses and Sky
• Polarizing filter on Overhead
• Polarizing filter on Projector

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Linear Polarization

• Long molecules.
• Electrons free to accelerate up and down the
  length, but dont have room to move along the
  width.
• Fence Analogy

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2 Polarizing Filters
Unpolarized Light has 50/50 chance of passing
through first filter.
Vertical Polarized Light has 0 chance of passing
through 2nd filter.
Conclusion 0 of the original light makes it
through the last filter.
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3 Polarizing Filters
Unpolarized
Here light is 45 deg Polarized 50 chance of
making it through next filter
Here light is V-Polarized 50 chance of making
it through 2nd filter
Conclusion 25 of the original light makes it
through the last filter.
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LCD screen
Screen Blocks Light
Screen Passes Light

• electricity orients easily movable molecules to
  produce a gradual change in the polarization
  which lets 90 of the original light make it
  through the third filter.

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Polarized Light

• Light can become polarized when it reflects off a
  surface such as glass or water or metal.
• Thus, polarized sun glasses get rid of glare.
• The sky is slightly polarized because the blue we
  see is light that has bounced off air molecules
  to us.

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Doppler ShiftRed Shift / Blue Shift due to motion

• Sound Demos
• Tuning Fork
• Car Horn Videos

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Moving Observer

• Demo Walking Students

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End
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Transverse vs Longitudinal.

• Only transverse waves can be polarized.
• Sound is a longitudinal wave so it has none of
  these polarization tricks work with sound.

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Ray Optics vs Wave Optics

• Ray Optics
• Light represented by a straight line.
• Easy way to represent what happens in lenses,
  mirrors, prisms, etc.

• Wave Optics
• Light represented by waves.
• More complicated, but some behaviors of light can
  only be explained using waves.