Dispersion of Light Revision Notes

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12.5Dispersion of Light


In these revision notes for Dispersion of Light, we cover the following key points:

  • What is dispersion of light?
  • Which light colour experiences dispersion?
  • Why does light experience dispersion?
  • What is an example of natural dispersion of light?
  • What are the primary colours of light?
  • What light colours are considered as secondary and tertiary?
  • How do we use prisms to produce dispersion of light?

Dispersion of Light Revision Notes

If the thickness of the refractive medium is constant, the white light will remain white during and after refraction. This is because all parts of the white light beam experience the same amount of refraction when passing through the second medium.

On the other hand, when the same white light beam falls on a refractive transparent medium with non-constant thickness, it will split into colours because they are incident at different angles and as a result, not all parts of the original beam travel the same distance through the refractive medium. Hence, the light waves of this beam will experience a deviation in proportion to the path they travel inside the refractive medium.

The phenomenon of splitting of visible light into its component colours is called dispersion.

Dispersion of light is caused by the change of speed of light ray resulting in a different angle of deviation for each wavelength.

Rainbow is a natural dispersion of light when refracting through small water droplets in air.

We cannot observe any dispersion when using monochromatic (one colour) light because only white light contains all colours.

Although we see seven colours when white light disperses through prisms, only three of them are primary, i.e. are not produced by combinations of other colours. These primary colours are red, green and blue.

Three other colours known as secondary colours are obtained by combining two primary colours. They are magenta, cyan and yellow. Thus, magenta is obtained by combining red and blue, cyan by combining green and blue, and yellow light is obtained by combining red and green.

The other colours not mentioned in the two above categories, are obtained by combining one primary and one secondary colour of light. These colours are known as tertiary or intermediate. For example, if you mix cyan and blue you obtain a new colour known as azure, by mixing blue and magenta you obtain violet etc.

Glass prisms can be used to change the direction of light and to produce dispersion. Right-angled prisms (with angles 45°-45°-90°) and equilateral prisms (with all angles equal to 60° for each corner) are the most common types of prisms.

If the light ray strikes this surface at an angle of incidence of 450 which is greater than the critical angle for glass (41.80), the total internal reflection occurs inside the glass.

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