What is Dispersion of light?

Neha Kishor
Updated on

The phenomenon in which the phase velocity of a wave depends on its frequency is called dispersion. Sometimes the term chromatic dispersion is used for specificity to optics.

A medium having this common property may be termed a dispersive medium. This term is used in the field of optics to describe light and other electromagnetic waves. Dispersion in the same sense can apply to any wave motion ie, acoustic dispersion in the case of sound and seismic waves, and in gravity waves. In optics, it is a property of telecommunication signals along transmission lines or pulses of light in optical fibre. Generally, dispersion translates into a loss of kinetic energy via absorption.

In optics, the familiar consequence of dispersion is the change in the angle of refraction of different colours of light.  It is seen in the spectrum produced by a dispersive prism and also in the chromatic aberration of lenses. Considering the design of compound achromatic lenses, here the chromatic aberration is largely cancelled. It uses quantification of a glass’s dispersion given by its Abbe number V. Here lower Abbe numbers correspond to greater dispersion over the visible spectrum. In practical applications such as telecommunications, the absolute phase of a wave is often not important. The only thing that is important here is the propagation of wave packets or “pulses”.

Every common transmission media also vary in attenuation as a function of frequency. It leads to the attenuation distortion and this is not dispersion. Although reflections at closely spaced impedance boundaries. It can produce signal distortion which furthermore aggravates inconsistent transit time as observed across signal bandwidth.

How dispersive prisms work

As light moves from one medium to another, it changes its speed. This speed fluctuation causes the light to be refracted. Then it enters the new medium at a different angle. The degree of bending of the light’s path depends on the angle of incidence. The refractive index of materials like  glass varies with the wavelength or colour of the light used. This phenomenon is known as dispersion. This causes the light of different colours to be refracted differently and to leave the prism at different angles. It also creates an effect similar to a rainbow. Dispersion can be used to separate a beam of white light into its constituent spectrum of colours.

Prisms will disperse light over a larger frequency bandwidth than diffraction gratings. It makes it useful for broad-spectrum spectroscopy. Additionally, prisms do not suffer from complications arising from overlapping spectral orders. A usual disadvantage of prisms is lower dispersion when it is compared to what a well-chosen grating can achieve.

Prisms are used for internal reflection on the surfaces rather than for dispersion. When the light inside the prism hits one of the surfaces at a sufficiently steep angle, total internal reflection occurs and all of the light is reflected. This makes a prism a useful substitute for a mirror in some situations.

Examples of dispersion in daily life:

formation of rainbow
  • Rainbow formation
  • Petrol poured on the water will show different colours.
  • Prism splits the light into different colours on passing through it.
  • Dispersion of colours in soap bubbles.
  • Dispersion of colours on CDs.
  • Dispersion could be seen from plastic scales.

Check your knowledge

When white light passes through a glass prism, its constituent colours (red, orange, yellow, green, blue, indigo, violet) travel at different speeds in the prism since the refractive index is colour dependent. This causes the dispersion of light.

The Violet colour of light deviates from the maximum during dispersion of white light by the prism as it has the least wavelength among all the colours.

The separation of light through a triangular prism is known as dispersion, and it works by splitting out the component colours blue, green, yellow, red, orange and violet. The reason for the individualization is due to the varying light wave frequencies and how they interact and bend once inside the prism.

During dispersion of light, the least dispersed is red colour.

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