Chapters: 3and 4. THREE MAIN LIGHT MATTER INTERRACTION Absorption: converts radiative energy into...

Chapters: 3and 4

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.

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.

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.

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.

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.

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.

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

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

SOURCES OF LINE

BROADENING

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

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

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

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

COMPARISON OF LINE SHAPES

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

SCATTERING PHASE FUNCTION

• Rayleigh scattering phase function is :

angle scattering is

cos14

3 2

rayP

ABSORPTION

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

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.

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

RADIATION LAWS

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

RADIATION LAWS

2

2

32

2

2

1:limit Jeans-Rayleigh .2

2

1:limit 1.Wiens

:are toionsApproximat

c

TkmvF

Tkhv

ehvc

mF

Tkhv

F

BrBBv

B

TkhvrBBv

B

BBv

B

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