Question

Explain emission and absorption spectra.

Answer 1

(i) Emission spectra: When the light emitted directly from a source is examined with a spectrometer, the emission spectrum is obtained. Every source has its own characteristics emission spectrum.
The emission spectrum is of three types. They are,
(a) Continuous spectrum, (b) Line spectrum and (c) Band spectrum.
(a) Continuous spectrum: It consists of unbroken luminous bands of all wavelengths containing all the colours from violet to red. These spectra depend only on the temperature of the source and is independent of the characteristic of the source.
Incandescent solids, liquids, carbon arc, electric filament lamps, etc., give continuous spectra.
(b) Line spectrum: Line spectra are sharp lines of definite wavelengths. It is the characteristic of the emitting substance. It is used to identify the gas.
Atoms in the gaseous state, i.e., free excited atoms emit line spectrum. The substance in atomic state such as sodium vapour lamp, mercury in mercury vapour lamp and gases in discharge tube give line spectra (Fig.)
(c) Band spectrum: It consists of number of bright bands with a sharp edge at one end but fading out at the other end.
Band spectra are obtained from molecules. It is the characteristics of the molecule. Calcium or barium salts in a bunsen flame and gases like carbondioxide, ammonia and nitrogen in molecular state in the discharge tube give band spectra. When the bands are examined with high resolving power spectrometer, each band is found to be made of a large number of fine lines, very close to each other at the sharp edge but spaced out at the other end. Using band spectra the molecular structure of the substance can be studied.

(ii) Absorption spectra: When the light emitted from a source is made to pass through an absorbing material and then examined with a spectrometer, the obtained spectrum is called absorption spectrum. It is characteristic of the absorbing substance.
Absorption spectra is also of three types. They are,
(a) Continuous absorption spectrum, (b) Line absorption spectrum and (c) Band absorption spectrum.
(a) Continuous absorption spectrum: A pure green glass plate when placed in the path of white light, absorbs everything except green and gives continuous absorption spectrum.
(b) Line absorption spectrum: When light from the carbon are is made to pass through sodium vapour and then examined by a spectrometer, a continuous spectrum of carbon arc with two dark lines in the yellow region is obtained as shown in figure.
(c) Band absorption spectrum: If white light is allowed to pass through iodine vapour or dilute solution of blood or chlorophyll or through certain solutions of organic and inorganic compounds, dark bands on continuous bright background are obtained. The band absorption spectra are used for making dyes.