Nếu chúng ta phân tán ánh sáng với máy quang phổ, chúng ta thấy tháng đó Nó là tạo thành rời rạc Nhiều dòng khí thải. Luận văn không đường vô cùng hẹp, có một mục tiêu và đo lường được chiều rộng hữu hạn hồ sơ. | 1 CHAPTER 9 OSCILLATOR STRENGTHS AND RELATED TOPICS Introduction. Radiance and Equivalent Width. If we look at a hot glowing gas we can imagine that we could measure its radiance in W m-2 sr-1. If we disperse the light with a spectrograph we may see that it is made up of numerous discrete emission lines. These lines are not infinitesimally narrow but have a finite width and a measurable profile. At any particular wavelength within the wavelength interval covered by the line let us suppose that the radiance per unit wavelength interval is I W m-2 sr-1 m-1. Here we are using the symbol I for radiance which is customary in astronomy rather than the symbol L which we used in chapter 1. We insist however on the correct use of the word radiance rather than the often too-loosely used intensity . We might imagine that we could measure I by comparing the radiance per unit wavelength interval in the spectrum of the gas with the radiance per unit wavelength interval of a black body at a known temperature or of any other body whose emissivity is known observed under the same conditions with the same spectrograph. The radiance I of the whole line is given by I JIkdk. In principle the integration limits are 0 and although in practice for most lines the integration need be performed only within a few tenths of a nanometre from the line centre. The radiance of an emission line depends among other things upon the number of atoms per unit area in the line of sight the column density in the initial . upper level of the line. You will have noticed that I wrote depends upon rather than is proportional to . We may imagine that the number of atoms per unit area in the line of sight could be doubled either by doubling the density number of atoms per unit volume or by doubling the depth of the layer of gas. If doubling the column density results in a doubling of the radiance of the line or expressed otherwise if the radiance of a line is linearly proportional to the column density .
