Dispersion: When a narrow beam of white light (sunlight or torch light) is passed through a triangular glass prism, it splits into a band of seven colours, as shown in Fig. The seven colours are in the order violet, indigo, blue, green, yellow, orange and red. The red colour is deviated the least while the violet colour is deviated the most. The colour sequence can be remembered by the acronym V1BGYOR.
The phenomenon of splitting of white light into its component colours on passing through a refracting medium such as a glass prism is called dispersion of light. The pattern of the coloured bands obtained on the screen is called spectrum.
Sir Isaac Newton was the first to use a glass prism to obtain the spectrum of sunlight.
Fig. Dispersion of white light into seven colours
Cause of dispersion of white light: Light rays of different colours travel with the same speed in vacuum. But in refracting media like glass, water, etc., lights of different colours travel with different speeds. The speed of violet colour is the least while the speed of red colour is the largest in glass. As a result, the refractive index of glass is largest for violet colour and least for red colour. The violet colour is deviated most while the red colour is deviated least on passing through the prism. Other colours are deviated by intermediate angles. So, different component colours of white light get dispersed on passing through a glass prism.
