Emission and Absorption Spectra

Spectra are mainly classified as emission and absorption spectra

Introduction to emission and absorption spectra:

Emission S[spectrum] :

It is the spectrum of light emitted by a substance. Emission spectra are of three different types

1) Continuous Spectrum : It is a spectrum that appears as an unbroken continuous band of colours from red at one end to violet at the other end. In such a spectrum we can not say where one colour ends and the other colour starts. A continuous spectrum is emitted when a solid or liquid is heated to very high temperatures. It is due to the thermal excitation of the molecules of the substance. Its examples are solar spectrum, carbon are spectrum, spectrum of light emitted by an incandescent bulb etc. Having problem with Units of Angular Momentum keep reading my upcoming posts, i will try to help you.

2) Band Spectrum : A spectrum in which different colours appear as bands separated by dark spaces is called band spectrum. Each band is sharply defined at one edge called head of teh band and fading off gradually at the other edge. When a band is observed by an instrument of high resolving power, it is found that each band consists of a large number of fine lines. The band spectrum is due to the excitation of the molecules of an element or a compound. Its examples are the spectra of CO2 , N2 obtained by filling the gas at low pressure in a discharge tube and passing electricity through it, the spectrum of the light emitted by Bunsen flame etc.

3) Line Spectrum : It is a spectrum of bright lines separated by dark spaces. The line spectrum is due to the excitation of the atoms of an element. A line spectrum is obtained when electrons excited to higher energy states make transition to lower energy states as explained by Bohr's theory of atomic structure. Some of the examples are the spectrum of the light emitted by hydrogen in a gaseous discharge tube, mercury  vapour lamp, sodium vapour lamp, the spectrum of the light  emitted by salt solutions of sodium, lithium, potassium, barium, calcium etc, placed in the non-luminous portion of Bunsen flame.

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Absorption spectra: emission and absorption spectra

When an absorbing substance is introduced between a source emitting continuous spectrum and a spectroscope it is found that some regions of the continuous spectrum are missing. This is due to absorption of light. Generally  absorption occurs whenever light passes through any transparent substance. In addition, certain substances strongly absorb light corresponding to certain parts of the spectrum which is called selective absorption.

Absorption spectrum : The spectrum of the light absorbed by a substance due to selective absorption is called absorption spectrum.  Absorption spectra are of two types.

1) Line Spectrum  :  When while light from an incandescent electric bulb is sent through a Bunsen flame fed with sodium salt, the transmitted light is found to be a continuous spectrum crossed by two close dark lines. These dark lines lie exactly in the same position as the emission lines of sodium and constitute the line absorption spectrum of  sodium vapour. Line absorption spectrum of an element is obtained when the temperature of  the source of light is higher than that of the atomic vapour through which it passes.

2) Band Spectrum : The absorption spectrum in the form of groups or bands of closely spaced lines is called band absorption spectrum. It is characteristic of molecular  gases and chemical compounds. In face molecular spectrum is better studied in absorption spectrum.