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CHAPTER 5 ABSORPTION, SCATTERING, (第五章吸收、散射、)
1
CHAPTER 5
ABSORPTION, SCATTERING, EXTINCTION
AND THE EQUATION OF TRANSFER
5.1 Introduction
As radiation struggles to make its way upwards through a stellar atmosphere, it may be
weakened by absorption and scattering. The combined effect of absorption and scattering is
called extinction. Scattering may simply be by reflection from dust particles. If the radiation
interacts with an atom, the atom may be excited to a higher energy level and almost immediately
(typically on a time-scale of nanoseconds) the atom drops down to its original level and emits a
photon of the same frequency as the one it absorbed. Such a process - temporary absorption
followed almost immediately by re-emission without change in wavelength - is probably best
described in the present context as scattering. Individual atoms in a stellar atmosphere generally
radiate dipole radiation; however, since many randomly oriented atoms take place in the process,
the scattering can be regarded as isotropic. If, however, the excited atom collides with another
atom before re-emission, the collision may be super-elastic; as the atom falls to a lower state, the
energy it gives up, instead of being radiated as a photon, goes to kinetic energy of the colliding
atoms. The radiation has been converted to kinetic energy. This process is absorption.
5.2 Absorption
To start with, let us suppose that the predominating mechanism is absorption with no scattering.
We can define a linear absorption coefficient α as follows. Let the specific intensity at some
level in an atmosphere be I. At a level in the atmosphere higher by a distance dx, the specific
intensity has dropped, as a
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