DISPERSION OF WHITE LIGHT
INTRODUCTION
Dispersion is the splitting up of white light into seven colours 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.
Recombination of Spectrum
The spectrum of colours can be reversed to its original light [fusion_builder_container hundred_percent=”yes” overflow=”visible”][fusion_builder_row][fusion_builder_column type=”1_1″ background_position=”left top” background_color=”” border_size=”” border_color=”” border_style=”solid” spacing=”yes” background_image=”” background_repeat=”no-repeat” padding=”” margin_top=”0px” margin_bottom=”0px” class=”” id=”” animation_type=”” animation_speed=”0.3″ animation_direction=”left” hide_on_mobile=”no” center_content=”no” min_height=”none”][colour]. This follows a simple procedure: The emergent rays from the prism which serve as incident ray for the second prism B, the rays are refracted and combined before coming out as a single ray of light which can be focused on the screen as a wavelength.
Newton’s Colour Disc
A Newton disc is a disc with segments in rainbow colours. When the disc is rotated, the colors fade to white; In this way Isaac Newton demonstrated that white light is a combination of the seven different colours found in a rainbow. A Newton Disc can be created by painting a disc with the seven different colours. A combination of red, green and blue in the circular disc will yield the same result. This is due to the phenomenon called persistence of vision. It can easily be board piece. It was an important discovery as it proves that light is not colourless, but has colour in it which together converge to give a faded white colour which we consider colourless. It was made by Isaac Newton. This property is based on the principles of dispersion of light.
Dispersion Power
The refractive index of the material of a prism is given by:
μ = sin (A + D/2)/sin (A/2)
Where A is the acute angle of the prism, D is the minimum deviation for small angle prism.
D = [μ – 1] A
for small prism A = α acute angle
D = d = minimum deviation
Then, d = [μ – 1]α
d = [μ – 1]α
The dispersion power (w) of the material of a small angle prism for blue and red rays is defined as the angular dispersive difference blue and red to its minimum deviation.
w = angular dispersive difference between red and blue
w = nb – nr/μ – 1
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