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Thermodynamics

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Solar Spectrum

The radiation received from the sun is similar to that of a perfectly black body. In a solar spectrum the wavelength corresponding to maximum energy is about 5000 Å for a surface temperature of 5700 K. The solar spectrum consists of a large number of dark lines called Fraunhofer lines. Those lines ere first observed by Wollaston in 1802 and later studied by Fraunhofer in 1814. These lines are observed in the whole length of the spectrum viz. ultraviolet, visible and infra=red. Fraunhofer measured the wavelengths of many these lines accurately and found that they occupied exactly the same positions as the bright lines emitted by various substances.

According to Kirchhoff’s law of radiation, any substance at a lower temperature will absorb the radiations of those wavelengths which it will emit when excited by electric discharge. Thus, the solar spectrum is an absorption spectrum and the Fraunhofer lines are the absorption lines of relatively cooler gases and vapours present in the earth’s atmosphere and the sun’s outer atmosphere. The central portion of the sun is called photosphere is the chromospheres which is at a much lower temperature than the photosphere. The positions of the Fraunhofer lines in the solar spectrum give the information regarding the nature of elements present in the sun’s atmosphere. The presence of more than sixty elements in the sun’s atmosphere is found this way. In fact, helium was first discovered by the study of the Fraunhofer lines in the solar spectrum before it had been isolated in the laboratory. The most prominent of the Fraunhofer lines are denoted by the letters of the alphabet. Some of the lines with their wavelengths and the elements responsible for their absorption are given below.