In short
Dispersion of light is the splitting of white light into its colours, because each colour travels at a slightly different speed in glass and so is bent by a different amount.
Violet light is slowed most in glass, so it is bent most. Red light is slowed least, so it is bent least. When white light passes through a glass prism, the colours leave at different angles and spread into a spectrum: red, orange, yellow, green, blue, indigo and violet. A rainbow is the same effect in raindrops.
A narrow beam of white light enters a prism. Violet leaves with the largest deviation and red with the smallest, so the red end of the spectrum is closest to the original direction of the beam.
Also searched as: dispersion of white light, colours of white light.
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