When Newton was observing stars using
astronomical telescope, he found that the stars were coloured near the fringes. He concluded that the colours observed were due to the nature of light but not due to the defect in lens. To prove this, he placed an equilateral prism in a path of narrow beam of light entering through a small aperture of a window in a dark room.
Emergent light from the prism was received in the screen. He found that emergent rays were spread out in bands of seven colours with red at one end merging gradually with orange, yellow, green, blue, indigo and violet.
The phenomenon of splitting of white light into its constituent colours is known as dispersion.
The medium which brings about dispersion is called dispersive medium.
Example: Prism is a dispersive medium. Vacuum or air is a non-dispersive medium.
The band of colours (wavelength) obtained is known as spectrum. Spectra are classified as
pure spectrum and
impure spectrum.
Pure Spectrum
It is the spectrum in which the constituent colours occupy their respective position (VIBGYOR) and are distinct. For example, spectrum obtained from prism and
spectroscope arrangement.
Impure Spectrum
It is a spectrum in which the bands of colours overlap. There are no well defined boundaries between the bands of colours.
Example: Rainbow
Rainbow Cause for Dispersion
Dispersion occurs because different colours (wavelengths) are refracted or deviated to different extents because speed of different colours of light is different in a given medium.
μ= Velocity of the light in air or vacuum/Velocity of light in air given medium
The refractive index of violet colour in glass is greater than the refractive index of red, or in other words, speed of violet is least in glass and the speed of red is the maximum
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