Physics Topics cover a broad range of concepts that are essential to understanding the natural world.
Why do Dark Lines Appear in the Solar Spectrum?
Solar Spectrum
Th e spectrum formed in the spectrometer by the light coming from the sun is called solar spectrum. In solar spectrum gener-ally seven colours from red to violet are found in a continuous fashion. So solar spectrum is a continuous emission spectrum.
Fraunhofer lines: If we carefully study the continuous spectrum of sunlight we will notice that a large number of dark lines cross the whole length of the spectrum. So, solar spectrum is actually a line absorption spectrum. The existence of these dark lines was first observed by Wollaston in 1802. In 1814 German scientist Fraunhofer made a systematic study of these lines.
These lines are known as Fraunhofer lines. He designated the major Fraunhofer lines by several letters from A to K. The lines A, B, C are in the red, D in the yellow, E and F in the green, G in the indigo, and H and K in the violet part of the spectrum. The positions of these dark lines are fixed and definite.?
Origin of Fraunhofer lines: The explanation of the dark lines was first given by Kirchhoff who, from several experiments, came to the conclusion that the vapour of an element absorbs those wavelengths at a particular temperature which it would emit if it were incandescent at that temperature. From this theory, the existence of dark lines is easily explained. The layer which we see when we look at the sun is called photo-sphere. This is generally considered the surface of the sun. The average temperature of the photosphere is 5700 K.
The sub-stances are in gaseous state in this layer. The temperature of the core is 15 × 106 K. The radiation from the core passes through the photosphere which are at a comparatively lower tempera-ture. So different wavelengths of light are absorbed here. Accord-ing to Kirchhoff’s law, white light emitted by the sun is robbed, while passing through the enveloping layer, of those waves which correspond to the waves that the element would emit if they were incandescent. In consequence of the absorption of these waves dark lines are observed.
Significance of Fraunhofer lines: From the study of Fraunhofer lines, it has been found that many elements are pres-ent on earth exist in the photosphere. Similarities have been found between fraunhofer lines and some of our known ele-ments spectra. So it can be inferred that these elements are pres-ent in sun’s photosphere. By analysing the solar spectrum the presence of about 70 elements (H2, Fe, Ca etc.) in the sun’s atmosphere has been confirmed. Before the existence of helium gas on the earth was discovered, its presence was predicted from the line absorption spectra.
Telluric lines: It is interesting to note that not all the dark lines across the continuous solar spectrum have been formed due to absorption by the various vapours present in the enveloping layer of the sun. When sunlight passes through the atmosphere of the earth, oxygen, water vapour etc. of the atmosphere absorb lights of different wavelengths. This causes a few more dark lines in solar spectrum. These dark lines are called Telluric lines. Telluric lines are not visible if solar spectrum is observed from a place at a height of 3 km or more from the surface of the earth, or from artificial satellite or from the moon.