This layer is also called the Sun’s atmosphere. Which is called chromospheres or reversing layer. The photosphere is surrounded by coloured gaseous envelope. Its temperature is about 20 million degrees Kelvin and it is assumed to emit white light. The central core of the Sun, called the photosphere is very hot.The lines could not be explained until Bunsen and Kirchoff in 1850 gave a satisfactory explanation based on the theory of absorption. The prominent lines in the visible spectrum were named by him as A, B, C, D……However, he could not explain why these lines occur at all. On account of their definite wavelengths he suggested that these lines can be used as standard wavelengths. He concluded that these lines occur in certain definite places in the solar spectrum and have definite wavelengths. Later Fraunhofer mapped these lines and measured their wavelengths. These lines are discovered by Wouaston in 1802. If we carefully examine, the continuous spectrum produced by the Sun, it is found to contain a number of dark lines spread over the entire length of the spectrum. Kirchoff’s law is applicable to the emission and absorption of light by any substance. arc lamp) and it starts absorbing the very same lines (wavelengths) which it was emitting.

But in the presence of strong arc lamp the temperature of sodium flames becomes less than that of the surroundings (i.e. Thus in the absence of carbon arc the temperature of sodium flame is higher than that of the surroundings and it emits two yellow lines. This means that sodium vapour which normally emits the two yellow lines when its temperature is higher than the surroundings, now absorbs the same wavelengths when its temperature becomes less than that of the surroundings. However, if an opaque screen is inserted between the arc and the sodium flame, the yellow lines once again reappear. Now if a sodium flame is interposed between the arc and the slit, two dark lines appear in the yellow region on the continuous spectrum exactly at the same position where the two sodium lines of wavelength 58 A. When the slit of a spectrometer is illuminated with light from carbon arc lamp, a continuous spectrum is observed. An opaque substance gives rise to a continuous absorption spectrum. Atoms in the gaseous state show line absorption spectra.Similarly, a solution of potassium permanganate gives five dark bands in the green region of continuous spectrum. Atoms in the gaseous state show line absorption substance, three dark bands are observed.Example: If a blue cobalt glass is used as an absorbing substance, three dark bands are observed.When a band is observed with an instrument of high resolving power, we find it consists of a large number of closely packed lines near the band head. The intensity of the band decreases gradually towards the other end, called the tail of the band. It consists of a large number of bands or strips of coloured light with dark gaps between them.Įach band is bright at one edge called the band head. Hence it is also known as molecular spectrum. If we know the spectra of different elements, it is possible to identify various elements present in a mixture.Įlectromagnetic radiation emitted by the molecules in the gaseous state form a band spectrum. Hence a line spectrum is considered as the signature o the element. The line spectrum of a given element cannot be ‘copied’ by any other element. The number of lines and their wavelengths depends upon the element. They give valuable information about the structure of the atom. Hence they are also known as atomic spectra. A line spectrum is produced due to the radiations emitted by the atoms in the gaseous state.

Emission spectra