Leigh wrote:
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I'm not sure what point Brian is making here. The chromosphere acts
as a filter, absorbing light from the photosphere so that it does
not get through to us. It does result in a reduction of flux at those
wavelengths. There is still some flux left at those wavelengths, from
the chromosphere itself, but it is much attenuated with respect to
that at nearby wavelengths. In fact this morning I will introduce my
students to the Saha equation so they can understand those lines in
some detail.
Leigh
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This reminds me of an experiment we used to do: Examine the spectrum
of an incandescent bulb shining through a sodium (NaCl) flame and then
into the spectrometer entrance slit. Vary the temperature of the
incandescent bulb with a variac and watch the sodium lines go from
emission (bright) to absorption (dark) as seen against the continuous
spectrum.