Spectrum of Light
The ordered arrangements of radiations according to wavelengths or frequencies is called Spectrum. Spectrum can be divided in two parts Emission spectrum and Absorption spectrum.
Emission spectrum : When light emitted by a self luminous object is dispersed by a prism to get the spectrum, the spectrum is called emission spectra.
Continuous emission spectrum
(i) It consists of continuously varying wavelengths in a definite wavelength range.
(ii) It is produced by solids, liquids and highly compressed gases heated to high temperature.
(iii) e.g. Light from the sun, filament of incandescent bulb, candle flame etc.
Line emission spectrum
(i) It consist of distinct bright lines.
(ii) It is produced by an excited source in atomic state.
(iii) e.g. Spectrum of excited helium, mercury vapours, sodium vapours or atomic hydrogen.
Band emission spectrum
(i) It consist of district bright bands.
(ii) It is produced by an excited source in molecular state.
(iii) e.g. Spectra of molecular H2, CO, NH3 etc.
Absorption spectrum: When white light passes through a semi-transparent solid, or liquid or gas, it's spectrum contains certain dark lines or bands, such spectrum is called absorption spectrum (of the substance through which light is passed).
(i) Substances in atomic state produces line absorption spectra. Polyatomic substances such as H2, CO2 and KMnO4 produces band absorption spectrum.
(ii) Absorption spectra of sodium vapour have two (yellow lines) wavelengths D1(5890Å) and D2(5896 Å)
