Light

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Light

• and other Electromagnetic Radiation

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Radiation

• A loaded term
• That which makes radiation dangerous is primarily
  its energy
• Energy is related to wavelength low frequency,
  long wavelength, low energy.
• Length of exposure, proximity to the source is
  also important.

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The Electromagnetic Spectrum

• Wavelengths ranging from 10,000 m (and longer) to
  0.000,000,000,000,01 m (and shorter).

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Danger!

• Pretty safe up through visible light (unless you
  stick your head in a microwave oven!).
• Pretty much dangerous from Ultraviolet and
  shorter.

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Sources of EM Radiation

1. Antennae
2. Atomic orbitals
3. Atomic nucleus

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Antennae

• Electrons (or any charged particle) emit electric
  fields.
• Moving charges make magnetic fields,
• so moving charges emit electromagnetic radiation.
• Mostly longer wavelength Marine radio to
  microwaves.

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Atomic Orbitals

• For all atoms hydrogen illustrated.
• When an atoms electron absorbs the right amount
  of energy, it will move to a higher energy level,
  a higher orbit about its nucleus.
• After a billionth of a second or so it falls back
  down to the initial or ground state.

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Photons

• As the electron drops back down to the ground
  state, it emits a photon (particle of light) with
  a wavelength directly dependent on the energy of
  the jump.
• These jumps to ground yield wavelengths too short
  to be seen (ultraviolet).
• However, when they land on the second level (n
  2), the result is a discreet spectra.

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Emission Spectrum
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Atomic Nucleus

• Very high energy photon from an atomic nucleus
  breaking apart.
• Takes some of that E mc2 for its energy.
• Hard X-Rays and Gamma rays.
• Even higher radiation comes from exploding stars
  (supernovae) cosmic rays.

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The Particle/Wave Duality

• Huygens 1650 light is a wave
• Newton 1700 light is a particle
• Young 1800 light is a wave
• Planck1900 light is a particle
• Its both! A particle of light (photon) has wave
  characteristics.

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Optics

• Geometric Optics
• Physical Optics

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Images

• Real (can be projected) or virtual (in the
  mirror)
• Erect (right-side up) or inverted
• Magnified (bigger) or diminished (smaller)

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Geometric Optics Mirrors

• Plane (flat) mirrors virtual image, erect,
  magnification of 1.
• Concave (curved inward). Like a makeup mirror.
  Inside the focal length (1/2 the radius of the
  curve) the image is virtual, erect and bigger.
  Outside its smaller, inverted, and real.
• Convex (curved outward). Passenger side mirror.
  Images are virtual, erect, and smaller.

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Index of Refraction

• Light travels at 186,000miles per second in a
  vacuum, but slower in everything else.
• In the process of slowing down as it enters
  water, glass, etc., the light beam bends inward.
• When it exits, it speeds up and bends away.
• The amount of bending depends on the Index of
  Refraction, the ratio of the speed of light in a
  vacuum to its speed in a medium.
• Can be used to make lenses, prisms, etc.

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Lenses

• Flat (like a window)
• Concave (diverging). Spreads light waves.
• Corrects near-sightedness
• Convex (converging). Focuses rays to a point.
• Corrects far-sightedness.

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Prisms

• Different wavelengths (colors) of light have
  different speeds in a medium. This causes them to
  bend different.
• White light is composed of all colors a prism
  separates them.

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Famous Album Cover
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Total Internal Reflection

• When light tries to escape from a medium, if the
  angle is too great then it is completely
  reflected back down.
• (the explanation requires trig!!)
• Circle of seeing for fish and divers looking
  up.
• The physics behind fiber optics.

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Physical Optics

• Diffraction
• Polarization

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Diffraction

• The bending of light around corners.
• When any wave encounters an edge, it bends around
  it.
• Light will spread out through a thin slit in
  otherwise opaque material.

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LASER

• Light Amplification by Stimulated Emission of
  Radiation.
• Useful for many things especially for
  diffraction demonstrations.
• Exactly one color of light that doesnt spread
  out (much).

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Interference

• When a slit causes light to bend, the distance
  from one edge to a far-off wall could be ½ a
  wavelength different from the distance from the
  other edge.
• Therefore, one wave may be up while the other
  wave may be down.
• The interfere destructively (cancel out)
• If both waves are NSync (sorry) then they
  interfere constructively (add up)

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Fringe Patterns

• The pattern of light and dark spots on the wall
  made by a LASER beamed through a slit.
• The reason? Diffraction and Interference!
• Gratings can also be used to separate colors,
  since the amount of bending depends on the
  lights wavelength.

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Polarization

• Light from most sources is disorganized.
• The plane of each wave is randomly oriented to
  each other. Think of many airplanes flying with
  different tilts to their wings unpolarized.
• A polarizing filter will allow only the correctly
  oriented waves through, lining them all up
  polarized.
• Two filters at 90o will block light altogether!
• Good in sunglasses for blocking glare, which is
  polarized.