Dr. Muzaffar Hussain Najar Spectrum by Dr. Muzaffar Hussain Najar Wednesday, June 05, 2019

SPECTRUM The impression produced on a screen when light from any source is diffracted through any diffracting medium is called spectrum. Emission Spectrum: The type of spectrum obtained when light emitted by an object is passed through a diffracting medium. Absorption Spectrum: When the light from a source emitting a continuous spectrum is first passed through an absorbing substance, and then observed, it is found that certain wavelengths are missing which leave dark lines or bands at their places. The observed spectrum is called absorption spectrum. Wednesday, June 05, 2019

EMISSION SPECTRUM Continuous Spectrum: When the source emitting light is an incandescent solid, liquid or gas at a high temperature, the spectrum so obtained is continuous. e.g. hot iron, hot charcoal. Line Spectrum: This is obtained when the light emitting substance is in the atomic state. e.g. spectrum obtained from light sources like mercury, sodium etc. Band Spectrum: This type of spectrum arises when the emitter in the molecular state is excited. The sources of band spectrum are carbon arc with a metallic salt in its ore and vaccum tubes etc. Wednesday, June 05, 2019

ABSORPTION SPECTRUM Continuous Spectrum: Arises when the absorbing material absorbs a continuous range of wavelengths. e.g., red glass absorbs all colours except red and hence a continuous spectrum is obtained. Line Spectrum: Sharp dark lines are observed when the absorbing substance is in a vapour or a gas phase. e.g., spectrum obtained from sun. Band Spectrum: When the absorption spectrum is in form of dark bands. e.g., aqueous solution of KMnO4 gives five absorption bands in the green region. It is produced by unexcited molecules. Wednesday, June 05, 2019

HYDROGEN SPECTRUM When an electric current is passed through hydrogen gas kept at low pressure, we get hydrogen spectrum which is found to consist of a number of lines in the visible, ultraviolet and infra-red regions. Each line of the spectrum corresponds to a particular frequency. The spectral lines can be grouped into five spectral series. Rydberg gave a general relationship for the wave number ν for the spectral lines. This formula, known as Rydberg’s formula, can be written as: Wednesday, June 05, 2019

HYDROGEN SPECTRUM Spectral Series Value of m Value of n Spectral region Lyman Series 1 2,3,4,……….∞ Ultraviolet region Balmer Series 2 3,4,5,……….∞ Visible region Paschen Series 3 4,5,6,……….∞ Infrared region Bracket Series 4 5,6,7,……….∞ Pfund Series 5 6,7,8,……….∞ Humphrey Series 6 7,8,9,……….∞ Far-Infrared region Wednesday, June 05, 2019

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