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Optical Physics - Spectrum


A collection of dispersed light giving its wavelength composition is called a spectrum. For example, when a white light is incident on a prism a spectrum of different colours from red to violet is observed.

Pure and impure spectrum: - in pure spectrum each colour given its sharp impression that is, lime spectrum is obtained. To achieve pure spectrum

(a) The light beam incident on the dispersing element should be parallel.

(b) The dispersed light should be for cussed in such a way that all the rays of a particular wavelength are collected at a place.

If the slit is wide different points of the slit produce separate spectra which overlap each other. The colour impression gets diffused due to overlap. Such a spectrum is called an impure spectrum.

Kinds of spectra: - broadly speaking spectra can be of two types’ emission spectrum and absorption spectrum. The light emitted by an object when it is suitably excited by heating or by passing an electric discharge and so on. If this light is dispersed by dispersing element to get the spectrum, it is called an emission spectrum. It reveals the information about the source material. Emission spectrum may be of three types. 

Continuous spectrum: - light emitted by a bulb; candle or a red hot iron piece whit light of continuously varying wavelengths in it. When such a light is dispersed, a bright spectrum continuously distributed on a dark background is obtained. This type of spectrum is termed as continuous spectrum.

Line spectrum: - when atoms (or substances in its atomic state) de – excite they produce bright colored lines, for example it gives D1 and D2 doublet 589 and 589.6 nm lines.

Band spectrum: - the molecular energy levels are generally grouped into several bunches, each bunch widely separated from the other and leveling bunch being close to each other. The wavelengths emitted by such molecules are also grouped. Each group is separated from each other. 

The wavelengths in a group are close to each other and appear as continuous. The spectrum looks like a separate band of colours.

Absorption spectrum: - when white light having all the wavelengths is passed through an absorbing material, the material may absorb certain wavelengths selectively (to get excited). These wavelengths will disappear when the transmitted light is dispersed. Bark lines or bands at the missing wavelengths one n otherwise bright continuous colored background appear. Such a spectrum is called absorption spectrum. It is of two types: line absorption spectrum and band absorption spectrum. When sunlight’s is superseding, certain sharply defined dark lines are seen. These lines are called Fraunhofer lines.  

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