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 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:

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.

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;

DOPPLER BROADENING Is as a result of random thermal motion of atoms. The cross section is given as: Doppler width is given by:

VOIGT PROFILE Is a profile representing combined effects of both lorentz and doppler broadening.

COMPARISON OF LINE SHAPES

SCATTERING PHASE FUNCTION

Rayleigh scattering phase function is :

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

Planck spectral distribution law is :

RADIATION LAWS

Wien’s displacement law: Stefan-Boltzmann law: