Introduction:
Dispersion is the splitting up of white light into seven colors on passing through a transparent medium like a glass prism. When a white light beam is passed through a prism, a band of seven colors are formed is known as spectrum of white light. The seven colors in the spectrum are red, orange, yellow, green, blue, indigo and violet respectively.
Dispersion of Light through Prism
The formation of spectrum of seven colors shows that white light is a mixture of seven colors (or seven colored light). The effect of transparent medium (like glass prism) is only to separate the seven colors of white light.
White light is a mixture of lights of seven colors: red, orange, yellow, green, blue, indigo and violet. The dispersion of light occurs because the angle of refraction (or the angle of bending) of lights of different colors is different when passing through the transparent medium (glass prism)
Pure Spectrum
Spectrum produced so far is impure because the rays overlap and the position of the spectrum also changes as the colour is not separated. A pure spectrum can be produced using a triangular prism-two converging lenses and a narrow slit of white light. The narrow slit is used to produce series of narrow coloured images, which minimizes the chances of overlapping colours, when incident rays are incidented on the converging lens which retracts the rays into a parallel beam and focus it on angle 60 triangular prism. The emergent rays are then focused on another converging lens to produce parallel of different colours on the screen at its focus. This produces a spectrum which does not overlap.
For the production of a pure spectrum, we require the following conditions:
(i) A narrow slit as a source of light; this produces series of narrow, coloured images, which minimizes the chances of overlapping colour.
(ii) A converging lens is placed with the slit at its focus, so that a beam of parallel light is produced.
(iii) A 60o prism for dispersion of the parallel beam.
(iv) A second lens for collecting the parallel beams of different colours, but this is not essential.
(v) A screen at the focus of the second lens on which the pure spectrum can be projected.
Impure Spectrum
Impure spectrum can be produced if only one converging lens is available. The image produced on the screen will not be pure, since there is absence of final converging lens to produce a parallel beam as that of pure spectrum. The converging lens focuses the refracted beam parallel to the prism which in turn is refracted and dispersed by focusing them on a screen. The prism must be placed at minimum deviation position to obtain a fairly impure spectrum…
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