An emission spectrum can be three types:
(a) Continuous spectrum: That emission spectrum which is obtained by continuously varying wavelength, is called continuous emission spectrum. In this case, when light is dispersed, a bright spectrum continuously distributed on a dark background is obtained. Light emitted from an electric bulb, a candle or a red hot iron piece comes under this category.
(b) Line spectrum: The atoms and molecules can have certain fixed energies.An atom or molecule, in an excited state, can emit light to lower its energy. Light emitted in such a process has certain fixed wave lengths. When such a light is dispersed, certain sharp bright lines on a dark background is obtained. Such a spectrum is called line emission spectrum.
For example, when electric discharge is passed through sodium vapour, they vapour emits light of the wavelength 589.0 nm and 589.6 nm. When this light is dispersed by a high resolution grating , one obtains two bright yellow lines on a dark background.
(c) Band spectrum : The wavelengths emitted by the molecular energy levels which are generally grouped into several bunches, are also grouped; each group being well separated from the other.The spectrum looks like separate bands of varying colours. Such a spectrum is called band emission spectrum.
