When Is a Spectral Line Not a Spectral Line?

Abstract

Electron transitions enable atoms to absorb or emit light. From what we have learned so far, the absorbed or emitted light is of a very specific wavelength which itself depends on the energy difference between the energy levels involved in the transition. This leads to the conclusion that absorption lines and emission lines also have very specific wavelengths—theHα line is at λ6563 after all.However, you only have to look at any astronomical spectrum to realise that spectral lines have a definite width, i.e. they spread out across a range of wavelengths.What’s more by careful examination it becomes clear that some lines, if they are absorption lines, are darker than others and some may even appear to divide up into several dark component lines connected by less dark regions. A spectral line clearly isn’t just a simple infinitely narrow line; the answer to the question posed in the title of this chapter is— ‘when it is a line profile’. All spectral lines have a line profile; in older books it was also sometimes referred to as the line contour.