Q.What are Fraunhofer lines? How are they useful in the identification of elements present in the Sun?

Passing light through a prism disperses it into a definite pattern of colours called a spectrum, and spectra are broadly classified as emission spectra (obtained by examining light directly from a self-luminous source) or absorption spectra (obtained by passing light through an absorbing medium), each further split into three sub-types. A continuous emission spectrum contains every visible wavelength with no gaps, produced by incandescent solids or a carbon arc; it is closely related to the black body spectrum, whose peak wavelength shifts to shorter values as an object's temperature rises (glowing red, then reddish-orange, then white as it gets hotter). A line emission spectrum consists of only a handful of sharp, bright lines at fixed wavelengths, produced by excited atoms of a gas (such as atomic hydrogen or helium); because each element's excited atoms emit only at their own specific wavelengths, this pattern of lines is a unique fingerprint that identifies the element. A band emission spectrum consists of many very closely spaced spectral lines that merge into distinct bands, with a sharp edge at one end and a fading edge at the other, arising when molecules (not just isolated atoms) are excited -- for example, the spectrum of ammonia gas in a discharge tube -- and is therefore characteristic of the molecule rather than of a single element, letting molecular structure be studied from its band spectrum. On the absorption side, a continuous absorption spectrum results when, for example, white light passed through a blue glass plate has every colour except blue removed. A line absorption spectrum results when light from an incandescent source passes through a cooler gas, which absorbs exactly the wavelengths its own atoms would otherwise emit if excited, leaving dark lines against an otherwise continuous bright background (for example, sodium vapour absorbing two lines in the yellow region from carbon-arc light). A band absorption spectrum results similarly when white light passes thro …