Dispersion is the splitting of white light into its separate colours, forming a spectrum.It happens because the refractive index of a material such as glass depends slightly on the wavelength (colour) of the light. Violet light, with the shortest visible wavelength, is slowed down most, so it has the highest refractive index and is refracted the most. Red light, with the longest wavelength, is slowed least and refracted the least.In a rectangular block the colours come out parallel again and overlap, so little splitting is seen. In a triangular prism the two refractions, one entering and one leaving, both bend the light the same way, so the colours spread out into a clear spectrum: red, orange, yellow, green, blue, indigo, violet.
Red: sin r = sin 45° / 1.51 = 0.468, so r = 27.9°.
Violet: sin r = sin 45° / 1.53 = 0.462, so r = 27.5°.
The two colours separate by about 0.4°, a small angle, which is why a prism is needed to see a good spectrum.Violet light also travels more slowly in the glass: v = c/n = 3.00 × 108 / 1.53 = 1.96 × 108 m s-1, compared with 1.99 × 108 m s-1 for red.Rainbows are caused by dispersion as sunlight refracts into and out of raindrops. Dispersion in lenses causes coloured fringes around images, called chromatic aberration.
Red: sin r = sin 45° / 1.51 = 0.468, so r = 27.9°.
Violet: sin r = sin 45° / 1.53 = 0.462, so r = 27.5°.
The two colours separate by about 0.4°, a small angle, which is why a prism is needed to see a good spectrum.Violet light also travels more slowly in the glass: v = c/n = 3.00 × 108 / 1.53 = 1.96 × 108 m s-1, compared with 1.99 × 108 m s-1 for red.Rainbows are caused by dispersion as sunlight refracts into and out of raindrops. Dispersion in lenses causes coloured fringes around images, called chromatic aberration.