The Propagation of Light

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The Propagation of Light

• Light in Matter
• Reflection
• Refraction
• Total Internal Reflection
• Fermats Principle

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Propagation of Light in VacuumRays
3
Light in Vacuum Rays
4
Polychromatic Rays
5
Light in MatterSpeed/Absorption/Scattering

• Speed of light in medium
• May be wavelength dependent
• Absorption
• Photon absorbed if energy matches excited state
  of atom
• In dense material, excited energy transferred via
  collisions to thermal energy before photon can be
  re-emitted
• Scattering
• Photon not absorbed but dipole oscillation of
  electron cloud occurs
• Radiates photon in some direction with same energy

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Rayleigh Scattering

• Scattering of light from molecules in air
• Depends strongly on wavelength
• Causes sky to be blue in appearance

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Light in Matter Refraction/Absorption

• Light in Vacuum
• Speed c3 x 108 m/s
• Absorption Coefficent 0

• Light in Matter
• Speed c/n
• n refractive index
• Absorption Coefficient
• energy decay after distance L e-2aL

Example glass fiber n1.5 a0.25dB/km0.0288/km
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Relative index of Refraction
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Molecular Model of Absorption
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Dispersion

• Frequency dependent permittivity
• Index of Refraction of a medium is frequency
  dependent

Material becomes polarized Frequency dependence
arises From mediums ability to respond
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Light Transmission through atmosphere
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Light in Metals
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Ideal Metals
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Non-Ideal Metals
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Law of Reflection
Do Demo
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ReflectionSpecular vs Diffuse
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Reflection
Specular Reflection
Diffuse Reflection
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Reflection in Plane Mirror
Virtual Image
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Reflection in Concave Mirror
If spherical mirror radius is small compared to
radius of curvature rays will come to a focus at
f r/2
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Spherical Mirror
As mirror diameter becomes large rays distant
from axis are out of focus
Do Demo
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Refraction

• As light crosses from a medium with index of
  refraction ni into another medium, nt , the speed
  of the light wave changes.
• The frequency of the light wave remains the same
• The wavelength must change in the transmitting
  medium

http//www.control.co.kr/java1/RefractionofLight/L
ightRefract.html
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Refraction
Enter a denser medium, bend towards the normal,
move slower
Enter a rarer medium, bend away from the normal,
move faster
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Total Internal Reflection
demo
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Total Internal Reflection
n2 sin(90)n1 sin(?c) http//freespace.virgin.net
/gareth.james/virtual/Optics/Refraction/refraction
.html
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Fermats Principle
Path that light travels Minimizes time For
reflection results in angle of incidence equal to
angle of reflection
Reflection
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Fermats PrincipleDerivation of Snells Law

• Light travels from A-gtB on path which needs least
  amount of time

n1 sin(?1) n2 sin(?2) fastest path
applet Derive on Whiteboard
A
B