= Absorption line
{title2=$I_{\rm line}<I_{\rm continuum}$}
An <absorption line> is a deficit of observed spectral <specific intensity> relative to the surrounding continuum. In a photosphere with <temperature> falling outward, increased line <opacity> makes photons emerge from cooler layers than the continuum. For an absorption-dominated <LTE> source, the <Planck function> then supplies a smaller line <specific intensity>. A foreground layer with source below the incident continuum gives another direct transfer description.
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