Atomic and molecular vibrations correspond to excited ：原子和分子的振动对应激.ppt

Scattering
Scattering fundamentals
Scattering can be broadly defined as the redirection of radiation out of the original direction of propagation, usually due to interactions with molecules and particles
Reflection, refraction, diffraction etc. are actually all just forms of scattering
Matter posed of discrete electrical charges (atoms and molecules – dipoles)
Light is an oscillating EM field – excites charges, which radiate EM waves
These radiated EM waves are scattered waves, excited by a source external to the scatterer
The superposition of incident and scattered EM waves is what is observed
Scattering geometry
Forward scattering
Backward scattering
(backscattering)
When does scattering matter?
Scattering can be ignored whenever gains in intensity due to scattering along a line of sight are pared to:
Losses due to extinction
Gains due to thermal emission
Usually satisfied in the thermal IR band and for microwave radiation when no precipitation (rain, snow etc.) is present
Also can be ignored when considering direct radiation from a point source, such as the sun
In the UV, visible and near-IR bands, scattering is the dominant source of radiation along any line of sight, other than that looking directly at the sun
Types of scattering
Elastic scattering – the wavelength (frequency) of the scattered light is the same as the incident light (Rayleigh and Mie scattering)
Inelastic scattering – the emitted radiation has a wavelength different from that of the incident radiation (Raman scattering, fluorescence)
Quasi-elastic scattering – the wavelength (frequency) of the scattered light shifts (., in moving matter due to Doppler effects)
More types of scattering
Single scattering: photons scattered only once
Prevails in optically thin media (τ<< 1), since photons have a high probability of exiting the medium (., a thin cloud) before being scattered again
Also favored in strongly absorbing media (ω<< 1)
Multiple scattering: prevails in op