The Solar photosphere is the region that is optically thick to visual continuum light; thus it is the lowest portion of the Solar atmosphere that can be observed with optical telescopes. Solar spectra show continua and emission and absorption lines. In the visible region, however, the sun shows an absorption line spectrum superimposed on a quasi-blackbody continuum formed in the solar interior. The absorption lines are formed as the continuum radiation passes through the cooler outer layers of the sun and through the earth's atmosphere. Each line corresponds to one (or more, in the case of several close, blended features) absorption line. Physical parameters of interest include temperature, pressure, abundances of various elements and compounds, magnetic field strength, and bulk velocity distribution.
Figure shows two scans of the solar spectrum in the region of the sodium Fraunhofer D lines at 5890 ? and 5896 ?. Several of the weaker features are due to water vapor in the earth's atmosphere, and show different strengths in these two scans because of the difference in humidity on the two days on which the scans were made.