When a solar spectrum is viewed, we can see a number of dark lines.
a. What are these dark lines called ?
b. How are they formed ?
c. What is their importance ?

Explanation

Fraunhoer lines
b. The central part of the sun is called photosphere, which is at an extermely high temperature of the order of `10^(7)`K it si surrounded by less donse luminous and highly compressed gases, and they form sun.s atmosphere. The photosphere emits radiations of all wavelengths. sorrounding the photosphere, There is another sphere of gases and vapours at a comparatively lower temperature (6000 K) called chromosphere. It contains certain terestrial element in the from of vapours. When continuous light of all wavelengths from the sun.s atmosphere passes through the chromosphere, certain wavelength are absorbed by the atmosphere passes through the chromosphere, certian wavelengths are absorbed by the elements in the chromosphere. this accounts for the presence of dark lines in the spectrum of the sun. These dark lines are called Fraunhofer lines. During a total solar eclipse, all the fraunhofer lines turn to bright lines.
.c The study of Fraunhofer lines establishes the presence of light gases like hydrogen , helium, oxygen , iron, sodium, calcium etc. In the outer envelope of the sun.