chapters: 3and 4. three main light matter interraction absorption: converts radiative energy into...
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Chapters: 3and 4
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THREE MAIN LIGHT MATTER INTERRACTION
Absorption:• converts radiative energy into internal energy
Emission:• converts internal energy into radiative energy
• Scattering; Radiative energy is first absorbed and then radiated.
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SCATTERING IN DIFFERENT MEDIA
Coherent scattering
• Scattering in which radiated wave has a definite phase shift with respect to the incident wave
• Observed when light is scattered by induced dipoles in a dielectric
Incoherent scattering
• No permanent definite phase relationship between radiated and incident waves and with in the individual scatterred fields.
• Observed when light is scattered by air or density fluctuations in an ocean.
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SCATTERING IN DIFFERENT MEDIA
Independent scattering
• Scattering in which radiated field from one scatterer doesnot interract with radiated field from a neighbour scatterer.
• Average spacing between scatterers should be several times their diameters.
• Example:Rayleigh scattering.
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SCATTERING IN DIFFERENT MEDIA
Conservative scattering
• Scattering process in which there is negligible absorption of incident field in the medium
Non conservative scattering
• Scattering process involving relatively much absorption of incident energy in the medium.
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SCATTERING IN DIFFERENT MEDIA
Elastic scattering
• No exchange of internal energy of the medium with the radiated field
• No change of frequency of incident wave upon scattering
Inelastic scattering
• Involves exchange of internal energy of the medium with that of the radiated field.
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SCATTERING IN DIFFERENT MEDIA
Optically thin medium:• is a medium with well separated particles that when each
receives direct radiation ,the diffuse radiation from all parts of the medium is negligible compared with direct radiation.
Optically thick media• This is a medium with a large number of scatteres that the
multiply scattered diffuse radiation is more compared with direct radiation.
• Common to planetary media.
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RESONANCE SCATTERING
rate damping is
oscillator offrequency natural is -
frequency driving is :where
4
4
4
o
22
2
v
v
vvcm
e
ooe
resn
•If the driving frequency is very close to the natural frequency. The scattering cross section for a damped simple oscillator becomes
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RAYLEIGH SCATTERING
• This occurs when driving frequency is less than the natural frequency of the oscillator.
• The cross section is given as:
2
2
24
6
1
ooe
rayn m
e
c
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SOURCES OF LINE
BROADENING
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NATURAL BROADENING
• This occurs when an isolated molecule is irradiated by an electroctromagnetic wave and the dampening of absorption line
produced is solely by the natural life time of excited quantum level.
levelupper of timelife natural is t
, t2
1
: asgiven is profile theof width Line
r
r
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PRESSURE BROADENING
• This is due to collisions between molecules that shortens the natural lifetime and broadens the line.
• Depends on number density and temperature. The broadened width is;
tempstandard is T
STPat number loschmidt -
o
L
oL
LL
n
Tn
TnSTP
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DOPPLER BROADENING
• Is as a result of random thermal motion of atoms.
• The cross section is given as:
• Doppler width is given by:
molecules. offrequency is
light of speed is
molecules of speedmean is V
: where
,
o
o
c
c
VooD
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VOIGT PROFILE
• Is a profile representing combined effects of both lorentz and doppler broadening.
sfrequencie allfor lorentz resembles
;1
wingsin thebehaviour like Lorentz
core linein result doppler
shows; 0
:ratio Damping
a
a
aD
L
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COMPARISON OF LINE SHAPES
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SCATTERING PHASE FUNCTION
angle scattering is
14
).;','(sin
4
)(cos
is ionnormalisat The
)( )(cos
)(cos)(cos
:section - xscatteringangular nomalised isIt
0
2
04
1
4
pdd
pdw
srdn
np
n
n
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SCATTERING PHASE FUNCTION
• Rayleigh scattering phase function is :
angle scattering is
cos14
3 2
rayP
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ABSORPTION
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ABSORPTION
• Energy selectivity is its outstanding characteristic.
• Energy attenuation due to absorption is dominant in near infrared and thermal infrared spectral ranges.
Absobed radiative enegy in these ranges causes:• Excitation of lattice vibrations,molecular vibrational
states,and intermolecular vibrations.
Absorbed radiative energy in UV and shorter wavelength leads to:
• Photodissociation,photoionisation
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ABSORPTION
Absorption in solids
• Conductors have a small gap between the energy bands and are higly absorbing and reflecting in visible and IR.
• Insulators have a bigger energy gap between the bands,so they are absorbing in UV
• Insulators are more or less transparent in visible and IR.
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ABSORPTION
Color & brightness of objects
• Selective absorption is responsible nearlly for all color of objects in the environment
• An exception rule to this is Rayleigh scattering
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RADIATION LAWS
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RADIATION LAWS
• Planck spectral distribution law is :
constantBoltzmann -
index refractive ofpart real -
constant splanck' is :Where
1)/exp(
2 3
2
2
B
r
B
rBB
k
m
h
Tkh
h
c
mF
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RADIATION LAWS
2
2
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2
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1:limit Jeans-Rayleigh .2
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1:limit 1.Wiens
:are toionsApproximat
c
TkmvF
Tkhv
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RADIATION LAWS
• Wien’s displacement law:
• Stefan-Boltzmann law:
ure temperatabsolute is T
radiation maximumat h wavelengtis
8.897,2
m
m mKT
body theof re temperatuis
constantBoltzmann -stefan is
4
T
TF BBv