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You might have seen and appreciated the seven colours of rainbow, colours of thin films, e.g., oil patches, colours seen while looking through a prism, etc.

Sir Isaac Newton was the first to show that white light is composed of several colours. He allowed rays of sunlight to enter a small aperture in a window, and projected the beam through a glass prism on to a wall in a darkened room. To his surprise, he discovered that the colours of a rainbow were projected on the wall, indicating that the white light had split up into its composite colours.


Dispersion is the splitting up of white light into its constituent colours when it is refracted through a prism. The seven main spectral colours are red, orange, yellow, green, blue, indigo and violet (Figure 9.30). The colours may be remembered with the word VIBGYOR, where each letter represents first letter of each colour, starting with violet.

Reason for Dispersion
Dispersion occurs because light of different colours (different wavelengths) travel with different velocities in a dense medium such as glass. Red light travels the fastest and therefore bends the least, while violet light travels the slowest and bends the most. In addition, the extent of deviation of a light ray by a prism depends on the angle of the prism and refractive index of the material of the prism. The more the refractive index, the more will be the angle of deviation (d).

Electromagnetic waves of all frequencies (and wavelengths) travel at a constant speed c = 3 × 108 m s−1  in vacuum. Hence, no dispersion can be observed in vacuum.

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