Resonant scattering of am intense beam of radiation by an arbitrary number of atoms contained in a volume whose dimensions are small as compared to the radiation wavelength is studied. It is seen that the scattered radiation is an equally spaced triplet whose middle frequency is equal to the incident frequency. Counting rates exact to all orders in incident intensity are obtained. Special cases of (i) atoms in ground state, (ii) atoms in excited state, (iii) assembly of atoms in super-radiant state, and (iv) assembly of atoms in thermal equlibrium are studied.
Solar radiation must pass at least once through the absorbing layer to reach the twilight zone of a layer of atoms scattering resonance radiation. The attenuation of the solar beam is calculated for resonance absorption and the resulting scattered intensity compared with the transparent layer model intensity for the same thickness of material. It is shown that in the case of sodium, the reduction in intensity for layers thicker than
109 atoms/
cm2 is too serious to permit the deduction of layer thickness from a simple theory which neglects imprisonment of resonance radiation.