When the light emitted directly from a source is examined with a spectrometer, the emission spectrum is obtained. Every source has its own characteristic emission spectrum.
The emission spectrum is of three types.
- Continuous spectrum 2. Line spectrum and 3. Band spectrum
It consists of unbroken luminous bands of all wavelengths containing all the colors 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.
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 in sodium vapour lamp, mercury in mercury vapour lamp and gases in discharge tube give line spectra (Figure).
Fig: Line spectrum of hydrogen
It consists of a 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 characteristic of the molecule. Calcium or Barium salts in a bunsen flame and gases like carbon−di−oxide, 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.