Dispersion of Light: The phenomenon of splitting of white light into its constituent colours, on passing through a prism is called dispersion of light. The band of colours so obtained is called a spectrum.
White light, e.g. light from the sun, when seen through a prism, appears coloured. Thus white light is split into different colours, on passing through a prism. The colours in the order of decreasing deviation are Violet, Indigo, Blue, Green, Yellow, Orange and Red, Fig. Obviously, the deviaition is maximum for violet colour and minimum for red colour.
Casue of Dispersion
The wavelength of red light (in the visible spectrum) is much larger than that of violet light and the wavelength of orange, yellow, green, blue and indigo colours lying in between red and violet light gradually decreases in the above order.
According to Cauchy's formula
µ = a + \(\frac{b}{\lambda^2} + \frac{c}{\lambda^4}\)
where λ is the wavelength of light and µ is the refractive index for the particular wavelength i.e. velocity of light is different for different wavelengths. As the wavelength of violet light is smaller than that of red light, the refractive index for violet light is greater than that for red light. i.e. since λv < λr
∴ µv > µr
We know that, the deviation δ for a small angled prism is given by δ = (µ - 1) A
So δv > δr
The other colours will deviate through angles having values between δr and δv.
