The Opacity of an Ionized Gas

Abstract

Theory of the absorption of radiant energy by free electrons in an ionized gas.—The classical theory of the optical properties of metals, supposed to contain free electrons, is applied to an ionized gas. As a result of collisions with molecules, vibrational energy supplied to the electrons by the electromagnetic field is transformed into energy of thermal agitation. The coefficient of absorption comes out proportional to ${\lambda }^2{p}^2$, where $\lambda$ and $p$ are the wave-length and pressure. There is also a scattering effect proportional to $p$ and independent of $\lambda$, but this is relatively unimportant except at pressures of ${10}^{-4\mathrm{}}$atm. or less. The equations developed are tentatively applied to a discussion of (1) opacity of the vapors of exploded wires. However Anderson reports an opacity 200 times greater than that roughly computed from theory, and varying inversely as $\lambda$ or ${\lambda }^2$. Evidently the subject requires further study. (2) Opacity of the solar atmosphere. Employing Saha's theory to calculate the ionization, coefficients are computed which indicate that electronic absorption may be an important cause of the opacity of the solar photosphere. It is concluded that light from regions where the pressure is greater than.01 atm. is cut off completely, so that all we see comes from a spherical shell of rarefied gas. (3) Opacity of giant stars. Computations made, assuming the relation between temperature and pressure given by Eddington's theory of stellar constitution, are found in rough agreement with Eddington's values of opacity.