Contents
By learning Physics Topics, we can gain a deeper appreciation for the natural world and our place in it.
What is Band Spectrum and Absorption Spectrum?
Any ray of light emitted from a source can be dispersed and a spectrum can be obtained. Spectra can be of different types.
Atomic theory helps to explain the formation of various spectra. [Discussion of this topic in detail will be discussed later.]
In spectroscopy, the different kinds of spectra obtained from dif-ferent sources of light are mainly of two types:
- emission spec-trum and
- absorption spectrum.
Emission Spectrum
Any body can be excited in such a way that it emits light. The nature of this light depends on the temperature and material of the body. The spectrum produced by this light in the spectro-scope is called emission spectrum. Spectroscope is an optical instrument used for creating and analysing a spectrum.
An object can be excited in many ways; e.g. by increasing its temperature, by transforming it to gaseous state, then by passing electric discharge through it at a low pressure, etc.
Spectrometer is an optical instrument with the help of which spectrum is created and analysed.
Emission spectra may be divided into three classes according to their formation:
- continuous spectrum,
- line spectrum and
- band spectrum.
Continuous spectrum: It is an unbroken band of light in which all the spectral colours from red to violet are present. There is no gap in this band from red to violet. In otherwords, this spectrum is not fragmented into various parts by black bands or lines. That is why it is called continuous spectrum.
Source: Solids or liquids, when heated to very high tem-peratures become incandescent and give rise to continuous spectrum; e.g., filament of an electric bulb, white hot molten metal etc. When an iron rod is heated to a high temperature it becomes red hot. On further heating, the rod turns white hot. White hot substances may exist in the solid or in the liquid state. The filament of an electric bulb is white hot, whereas the fila-ment of an electric heater is red hot. Incandescent gaseous substances at extremely high pressure also show this type of spectrum, an example being the solar spectrum.
Characteristics: The intensity of the yellow portion of the continuous spectrum is maximum and intensity diminishes gradually as we move towards the red and the violet extremities of the spectrum.
From a continuous spectrum we can more or less predict the temperature of the source. However, the chemical composition of the source cannot be known from this spectrum because dif-ferent materials of varying compositions produce similar contin-uous spectra.
Line Spectrum: It consists usually of a number of bright lines separated by dark spaces. Hence, it is called line spectrum. Obviously, light of all wavelengths are not present in this spec-trum.
Source: Atoms of vapours or gaseous elementary substances, in their incandescent states produce line spectra. A bit of metallic salt such as NaCl when introduced into a colourless bunsen flame gives rise to this kind of spectrum. A vacuum tube containing a gas and made luminous by an electric discharge with an induction coil, also gives rise to a number of isolated bright lines. Vapour-lamps also give rise to line spectra. Colour as well as the position of these lines differ with different gases.
Characteristics: Line spectrum is the characteristic of an atom of an element. Any element can be identified by observing its line spectrum. The colour and position of the lines of line spectrum of every element are definite and fixed.
In the sodium line spectrum two yellow lines occur very close to each other. These are called the D1 line of wavelength 5896 A and D2 line of wavelength 5890 Å. It has been proved experi-mentally that no other element, except sodium, produces a spectrum with such yellow lines. In hydrogen line spectrum, a red, a blue and two violet lines are present. On analysing and studying the line spectra therefore, we can identify the sub-stances and also gather various information regarding the atomic structures of elements.
By analysing the spectra of a star we can know what elements are present there. Often, by observing the brightness of the lines, we can even guess the quantity of the elements.
Band Spectrum: It is a spectrum comprising a number of bands of colours, arranged one after the other with some dark gaps in between. As this spectrum consists of bands of light it is called band spectrum. It is obvious that light of all wavelengths are not present in this spectrum.
Source: The light emitted from the molecule of an incan-descent gaseous substance produces band spectrum. Such a spectrum can be obtained by sending electric discharge into a discharge tube containing oxygen or nitrogen gas at low pressure. Chemical compounds such as cyanogen, nitric oxide also give rise to band spectra.
Characteristics: The bands in this spectrum are sharply defined on one side and get diffused on the other side. The por-tion of the band which is bright is distinctly demarcated. This is called the band head. However, as we move in the other direc-tion the brightness of the band diminishes and there is no distinct line; this part is called the band tail. On analysing these bands with a high resolving power instrument, it is observed that each band is a collection of some discrete lines packed very close to each other. Towards the band head the lines occur very close to each other. But, as we move towards the band tail the space between the lines goes on increasing.
Band spectrum is a light spectrum emitted from the atom of ele-ment or compound. By analysing band spectrum considerable information regarding structure of molecule can be obtained.
Absorption Spectrum
When white light passes through a transparent material, light of one or more colours present in white light may be absorbed by that transparent material. The spectrum produced in the spec-trometer by light of other remaining colours emitted from the transparent material is called absorption spectrum. Dark bands are seen in the spectrum due to absorption of some colours (or wavelengths).
Absorption spectrum may be divided into two types according to their formation:
- line absorption spectrum and
- band absorption spectrum.
Line absorption spectrum: It is a spectrum in which a number of dark lines separated by some distance are seen.
Source: It is the result of selective absorption of some colours (or wavelengths) by some substances (monoatomic gas or vapour). A substance which is capable of emitting some wave-lengths, absorbs the same wavelengths when light from an exter-nal source at higher temperature is made to pass through it. Sodium flame (about 900°C) gives a line spectrum. When light from a source (electric arc at 3500 °C) at higher temperature is passed through it, the flame absorbs the same wavelengths, thereby producing dark lines in their place.
Characteristics: The dark lines are parallel to each other and coloured region exists between two consecutive lines. Like line emission spectrum, the positions of the dark lines in line absorption spectrum depend on the nature of the gas. So by observing the position of dark lines in line absorption spectrum, absorbent substances can be identified i.e., line absorption spec-trum expresses the characteristics of the atoms of the absorbent substance.
Band absorption spectrum: It is a spectrum in which a large number of dark lines (due to absorbed wavelengths) group together and are so close to each other that it appears as though a number of dark bands cross the spectrum. The black bands are called absorption band.
Source: This is obtained when light giving a continuous spectrum, coming from a source at higher temperature passes through polyatomic gases (O2, N2, CO2, etc.) at a lower tem-perature and pressure.
Characteristics: In absorption spectrum a large number of dark lines group together. In this spectrum, dark bands are formed at those places where colour bands are seen in emission spectrum. Because the colour bands, emitted (radiated) by the gas-molecules in its incandescent state, are absorbed by them from the white light, this spectrum expresses the characteristics of the molecules of the absorbent substance.