Emission and Absorption of Light and Quantum Mechancial Condtion

In this article we are going to discuss classification of spectra in detail. Quamtum mechanics shows that the energy levels of all systems are quantised and are designated by the appropriate quantum numbers. Now, let us consider how a spectrum arises. Consider two energy levels  E1 and E2 as shown in fig. Where a photon of frequency ν falls on a molecule in the ground state and its energy hν is exactly equal to the energy difference ΔE ( E2-E1 ) between the two molecular energy levels, then the molecule undergoes a transtion from the lower energy level to the higher energy level with the absorption of the photon of energy hν.

The spectrum thus obtained is called the absorption spectrum, on the contrary, if the molecule falls from the excited state of the ground state with the emission of a photon of energy hν, the spectrum thus obtained is called emission spectrum.

Classification of spectra and quantum mechanical condition

Classification of spectra and quantum mechanical condition

Energy ( E=hν= hc/λ) may be measured in electron volt (eV), calorie (cal) or ergs (erg). Inter conversion of different  energy units is shown in table.

Classification of spectra and quantum mechanical condition

Classification of spectra

Spectra is classified in following ways-

1. Emission spectra 

When a substance is heated up to incandescent or a gas electrically discharged, then radiation, are emitted and when these radiations are dispersed in a spectroscopy, the obtained spectrum is called emission spectra. 

2. Absorption spectra

When radiations obtained from a source are absorbed by a medium (solid, liquid or gas), and transmitted radiations are analysed by a spectrosope, the obtained spectrum is known as absorption spectrum. In such spectra dark lines or bands are obtained on the bright background. These dark lines or bands shows that certain part of radiation have been absorbed by the medium.

Emission and absorption spectra are further classified according to their structure

(1.) Continuous Spectrum

(2.) Discontinuous Spectrum

(1.) Continuous Spectrum 

When sun light or radiation from an incandescent material are analyzed by a spectroscope, different bands are viewed on the screen. These bands are diffused in each and other can not be distinguished separately. Such type of spectrum is known as continuous emission spectrum.

If a red coloured glass is placed in the path of visible light then glass absorbs all radiations expect red one and then obtained is known as continuous absorption spectrum.

(2.) Discontinuous spectrum

These spectra are not continuous and further be classified into two types

(A) Line spectrum

(B) Band spectrum

(A) Line or Atomic spectrum

When the radiation emitted by the vapours of an element are viewed through spectroscopy, bright lines are observed in a series. These lines correspond specific radiations emitted by the vapours of that element. Thus this type of spectra is called atomic or line spectra. And For example, the spectrum obtained from the sodium vapours have two bright lines at 5890 and 5896 Å wavelength. Since no two elements give identical lines, therefore line spectra are the characteristic properties of atoms. Thus line spectra helps in the detection of elements.

(B) Band or Molecular Spectrum

Band Spectrum is in the form of the group of lines whose one end of is sharp with a clear boundary which goes on diffused at other end. If these bands are observed through a powerful microscope, they appear ij the from of groups of many lines. If the substance which give emission band spectrum is heated upto high temperature the band spectrum disappers. Hence, It means band spetra are produced by molecules which dissociate into atoms on heating. That is why band spectra are also known as molecular spectra. They are also of both types i.e. emissiom and absorption spectra.


In this particular article, theory of classification of spectra we have studied Emission and Absorption of Light and Quantum Mechancial Condtion. We have studied about the classification of spectra in easiest way possible.

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