WHAT IS SPECTRUM AND IT'S KINDS

WHAT IS SPECTRUM AND IT'S KINDS

When light coming from a source is dispersed by a prism or any other dispersing element, light of different wavelengths are deviated through different angles and get separated. Such a dispersed light may be received on a screen, on a photographic plate or it may be viewed directly by the eye. A collection of dispersed light giving its wavelength composition is called a spectrum.As a very simple demonstration, let white light fall on a prism and collect the transmitted light on a white wall or a white paper. A spectrum consisting of different colours from red to violet is obtained.

KINDS OF SPECTRA

There are two types of spectra

A.  Emission Spectra

Light is emitted by an object when it is suitably excited by heating or by passing an electric discharge. When a light beam emitted by such a source is dispersed to get spectrum, it is called an emission spectrum. An emission spectrum carries information about the source material. An emission spectrum can be of three types:

(a) continuous spectrum 

Quite often a source emits light which has continuously varying wavelengths in it. electric bulb, a candle or a red hot iron piece emits light of this type. When such a light is dispersed, a bright spectrum continuously distributed on a dark background is obtained. The colours gradually change and there are no sharp boundaries in between. Such a spectrum is known as a continuous emission spectrum.

(b) line spectrum

All objects are made of atoms and molecules. The atoms and molecules can have certain fixed energies. An atom or a molecule having the lowest possible energy is said to be in its ground state, otherwise, in an excited state. An atom or molecule, in an excited state can emit light to lower its energy. Light emitted in such a process has certain fixed wavelengths. The light emitted by one kind of atoms generally have widely separated wavelength components (figure3.1a). When such a light is dispersed, we get certain sharp bright lines on a dark background. Such a spectrum is called line emission spectrum. It carries information about the atoms of the source. For example, when electric discharge is passed through sodium vapour, the vapour emits light of the wavelengths 589.0 nm and 589.6 nm. When dispersed by a high resolution grating, one obtains two bright yellow lines on a dark background.

(c) Band spectrum

The molecular energy levels are generally grouped into several bunches, each bunch widely separated from the other, and the levels in a bunch being close to each other. Thus, the wavelengths emitted by such molecules are also grouped, each group being well separated from the other. The wavelengths in a group are close to one another and appear as continuous. The spectrum looks like separate is called a band emission spectrum. Figure(3.1b) shows schematically the production and appearance of band spectra.

B. Absorption spectrum

When white light having all wavelengths is passed through an absorbing material, the material may absorb certain wavelengths selectively. When the transmitted light is dispersed, we get dark lines or bands at the position of the missing wavelengths superposed on an otherwise bright continuous coloured background (figure 3.2).

The missing wavelengths provide information about the absorbing materrial. Such a spectrum is called an absorption spectrum. 

Absorption spectrum may be of two types depending on the absorbing material and the conditions, such as temperature, of the experiment.

(a) Line Absorption spectrum

light may be absorbed by atoms to take them from lower energy states to higher energy states. In this case, the missing wavelengths are widely separated and we get sharp dark lines on a continuous bright background. Such a spectrum is called a line absorption spectrum. When light coming from the sun is dispersed, it shows certain sharply defined dark lines. This shows that certain wavelengths are absent. These missing lines are called Fraunhofer lines.

(b) Band absorption spectrum

If light is absorbed by molecules of the absorbing material, exciting them from lower energy to higher energy states, the missing wavelengths are grouped into bunches. Thus, when the transmitted light is dispersed, we get separate dark bands on a continuous bright background. Such a spectrum is called a band absorption spectrum. Light passing through hydrogen gas at moderate temperature or through certain solutions of organic and inorganic compounds shows such a spectrum.