THREE MAIN LIGHT MATTER INTERRACTION

Transcription

1 Chapters: 3and 4

2 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.

3 Coherent scattering SCATTERING IN DIFFERENT MEDIA 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.

4 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.

5 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.

6 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.

7 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.

8 RESONANCE SCATTERING If the driving frequency is very close to the natural frequency. The scattering cross section for a damped simple oscillator becomes σ n res = e ۲ ٤m e ε o cπ [ γ/٤π v o v ۲ γ /٤π ۲ ] where: v is driving frequency v o - is natural frequency of oscillator γ is damping rate

9 RAYLEIGH SCATTERING This occurs when driving frequency is less than the natural frequency of the oscillator. The cross section is given as: σ n ray = ۱ ٦π ω c ٤ e ۲ m e ε o ω o ۲ ۲

10 SOURCES OF LINE BROADENING

11 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. Line width of the profile is given as : α= ۱ ۲πt r, t r is natural life time of upper level

12 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; α L =α L STP n T n L T o n L - loschmidt number at STP T o is standard temp

13 DOPPLER BROADENING Is as a result of random thermal motion of atoms. The cross section is given as: Doppler width is given by: α D = ν o V o, c where : V o is mean speed of molecules c is speed of light ν o is frequency of molecules.

14 VOIGT PROFILE Is a profile representing combined effects of both lorentz and doppler broadening. Damping ratio: a= α L α D a ۰ shows; doppler result in line core Lorentz like behaviour in the wings a ۱; resembles lorentz for all frequencies

15 COMPARISON OF LINE SHAPES

16 SCATTERING PHASE FUNCTION It is nomalised angular scattering x- section: p cosθ = nσ n cosθ sr ۱ d ωσ n cosθ n ٤π The normalisation is ٤π dw p cosθ ٤π ۲π = ۰ Θ is scattering angle π dφ ۰ dθ sin θ p θ ', φ'; θ. φ ٤π =۱

17 SCATTERING PHASE FUNCTION Rayleigh scattering phase function is : P ray Θ = ۳ ٤ ۱ cos۲ Θ Θ is scattering angle

18 ABSORPTION

19 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

20 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.

21 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

22 RADIATION LAWS

23 RADIATION LAWS Planck spectral distribution law is : F BB ν = m ۲ r ۲πhν ۳ c ۲ [exp hν/k B T ۱] Where: h is planck's constant m r - real part of refractive index k B - Boltzmann constant

24 RADIATION LAWS Approximations to F v BB are: ۱. Wiens limit :hv /k B T >> ۱ F BB v m ۲ r c ۲ ۲π hv۳ e hv /k BT ۲. Rayleigh-Jeans limit: hv / k B T << ۱ F v BB ۲πv۲ m r k B T c ۲

25 RADIATION LAWS Wien s displacement law: λ m T =۲,۸۹۷.۸µ mk λ m is wavelength at maximum radiation T is absolute temperature Stefan-Boltzmann law: F v BB =σt ٤ σ is stefan-boltzmann constant T is temperature of the body