week 10 – more e&m theory, attenuation, color
TRANSCRIPT
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Week 10 – More E&M theory, attenuation, color
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Dispersion model for Dielectric
Dispersion-model.xmcd
Resonant frequencies at 1012 and 5*1012 radians/sec
damping frequencies at 1012/10 and 5*1012 /10 radians/sec
Oscillator strengths equal for both.
22
2 20
( ) j
je oj j
fNqn
m i
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Dispersion model for Dielectric
Dispersion-model.xmcd
damping frequencies at 1012/10 and 5*1012 /10 radians/sec
How does refractive index change with damping frequency?
damping frequencies at 1012/2 and 5*1012 /10 radians/sec
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Dispersion model for Dielectric
Dispersion-model.xmcd
Equal oscillator strengths
How does refractive index change with oscillator strength?
Oscillator strength of high frequency 3 times low frequency
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Dispersion model for Metal
Metal-Dispersion-model.xmcd
Note: for Electromagnetic frequencies below plasma frequencies,And imaginary refractive index LARGE.
For frequencies above plasma frequency,
And real refractive index approaches UNITY.
Plasma frequency set to be 1012 radians/sec
0/ 0
/ ~ 1o
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Negative Refractive Index Materials
Snell’s Law with negative refractive index… so called ‘Left Handed Materials’
‘Cloaking’ devices
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Focusing and Absorbing example
You can also design metamaterials which are perfect absorbing material (in a spectral range) …. Good for stealth technology. Energy from radar absorbed rather than reflected.
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Negative Refractive Index MaterialsTime Averaged Poynting Vector is ANTIPARALLEL to phase velocity.
E, B, and k follow a LEFT HANDED rule
E
Bk
Right-handed ‘NORMAL’ wave
Negative Refactive Index wave
E
B
k
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What do metamaterials look like?
Microwave range – combination of metallic split rings (for magnetic permeability control) and metallic lines (for permittivity E field controll)
Size of individual structures SMALLER than wavelength of electromagnetic radiation
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Subwavelength Structures – Photonic Crystal
Pillars of GaAs
Light in
Light out
wavelength
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Other methods of ‘cloaking’
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Color from ScatteringOpal structures – (interference from scattered light from small structures)
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Primary ColorsRed Green Blue - RGB
What is this due to?
Primary color set NOT unique. Typically use RGB since these are the phosphors used to generate colors in TV sets/ monitors. RGB also correspond (roughly) to the photoreceptors in the human eye
James Clerk Maxwell
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ADDITIVE combinations of RGB
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Human Detection of Light• Two types of color receptors in the human eye– Rods (best for dim light)– Cones (best for bright light)
Normalized responsivity spectra of human cone cells, S (short), M (medium), and L (long) types
Wavelength (nm)
Resp
onsi
vity
Rod sensitivity compared to Cones
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SUBTRACTIVE combinations (ie. Transmission)
STARTING with WHITE light, REMOVE the blue color (by absorbing blue) and the resulting color appears as yellow (combination of Red and Green)
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Additive vs Subtractive Colors
RGB cyan, magenta and yellow (CMY)
Complimentary colors – Absence (subtraction or absorption) of green light from ‘white’ light gives purple (magenta).
Eg. Amount of printed magenta ink determines how much green light is absorbed (not reflected) from page
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Transmission colors
Overall transmission is the MULTIPLICATION or product of individual multiplications