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Waves, Light & Quanta Tim Freegarde
Web Gallery of Art; National Gallery, London
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Rainbows
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Rainbows i x r r r i r r r i
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Rainbows i x r r r i r r r i
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Rainbows i x r r r i r r r i
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Rainbows
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Rainbows
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Rainbows
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Sinusoidal waves z simple harmonic motion circular motion where
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Sinusoidal waves at , wavenumber spectroscopists’ wavenumber
wavelength
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Sinusoidal waves at , angular frequency frequency period
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Suggested textbooks Hecht Optics Pedrotti & Pedrotti
Introduction to Optics Smith, King & Wilkins Optics and Photonics Show books
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Birefringence asymmetry in crystal structure causes two different refractive indices opposite polarizations follow different paths through crystal birefringence, double refraction
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Optical polarization light is a transverse wave: perpendicular to
for any wavevector, there are two field components any wave may be written as a superposition of the two polarizations
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Linear dichroism conductivity of wire grid depends upon field polarization electric fields perpendicular to the wires are transmitted fields parallel to the wires are absorbed WIRE GRID POLARIZER
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Malus’ law amplitude transmission intensity transmission
WIRE GRID POLARIZER
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Linear dichroism crystals may similarly show absorption which depends upon linear polarization absorption also depends upon wavelength polarization therefore determines crystal colour pleochroism, dichroism, trichroism TOURMALINE
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Polarization in nature
CUTTLEFISH (sepia officinalis) the European cuttlefish also has polarization-sensitive vision … and can change its colour and polarization! MAN’S VIEW CUTTLEFISH VIEW (red = horizontal polarization)
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Characterizing the optical polarization
wavevector insufficient to define electromagnetic wave we must additionally define the polarization vector e.g. linear polarization at angle
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Categories of optical polarization
linear (plane) polarization non-equal components in phase circular polarization equal components 90° out of phase elliptical polarization all other cases
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Polarization notation
circular polarization RCP plane of incidence right- or left-handed rotation when looking towards source traces out opposite (right- or left-) handed thread perpendicular parallel linear (plane) polarization parallel or perpendicular to plane of incidence plane of incidence contains wavevector and normal to surface
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Circular dichroism absorption may also depend upon circular polarization SCARAB BEETLE LEFT CIRCULAR POLARIZED LIGHT RIGHT CIRCULAR POLARIZED LIGHT the scarab beetle has polarization-sensitive vision, which it uses for navigation the beetle’s own colour depends upon the circular polarization
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Optical activity (circular birefringence)
optical activity is birefringence for circular polarizations CH2 CH3 H CH2 CH3 H an asymmetry between right and left allows opposing circular polarizations to have differing refractive indices l-limonene (orange) r-limonene (lemon) optical activity rotates the polarization plane of linearly polarized light CHIRAL MOLECULES may be observed in vapours, liquids and solids
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Birefringence asymmetry in crystal structure causes polarization dependent refractive index ray splits into orthogonally polarized components, which follow different paths through crystal note that polarization axes are not related to plane of incidence
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Optical polarization light is a transverse wave: perpendicular to
for any wavevector, there are two field components any wave may be written as a superposition of the two polarizations
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Electromagnetic waves
light is a transverse wave: perpendicular to Faraday Ampère
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Dielectrics z atomic electrons move in response to electric field
resulting atomic dipole radiates field which adds to original z Faraday Ampère
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Birefringence asymmetry in crystal structure causes polarization dependent refractive index ray splits into orthogonally polarized components, which follow different paths through crystal note that polarization axes are not related to plane of incidence
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Linear polarizers (analyzers)
o-ray birefringence results in different angles of refraction and total internal reflection 38.5º e-ray many different designs, offering different geometries and acceptance angles e-ray o-ray s-ray a similar function results from multiple reflection p-ray
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Waveplates (retarders)
at normal incidence, a birefringent material retards one polarization relative to the other linearly polarized light becomes elliptically polarized WAVEPLATE
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Compensators a variable waveplate uses two wedges to provide a variable thickness of birefringent crystal adjust a further crystal, oriented with the fast and slow axes interchanged, allows the retardation to be adjusted around zero variable fixed with a single, fixed first section, this is a ‘single order’ (or ‘zero order’) waveplate for small constant retardation SOLEIL COMPENSATOR
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