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Prism Dispersion


White light is a composition of seven colours belonging to a different spectrum of electromagnetic radiation. Hence, we refer to white light as sunlight or even light from an LED bulb. Since white light is a composition of seven colours, it can be split into individual colours. This phenomenon of splitting white light into its seven colours when it passes through an object is known as dispersion.

These seven colours are violet, indigo, blue, green, yellow, orange, and red. We see dispersion occurring naturally in nature. Rainbow is the result of dispersion as well. Dispersion plays a significant role in other situations as well. For example, in optical fibres, group velocity dispersion spreads the pulses, thus, degrading the signal strength over a long distance.

In dispersion, the phase velocity of a wave is dependent on its frequency. Artificially, dispersion through a prism can be understood clearly.

Prism dispersion

The dispersion phenomenon depends upon the material, the speed of light and the type of light that we allow to pass through the prism. If the material is denser, then the dispersion will be lower. If the speed of light is faster, the dispersion will be faster. So light belonging to different electromagnetic spectrums will be dispersed differently or maybe in only two or three colours.

The prism's shape is made so that it bends the white light when it passes through the prism. The white light bends more than passing through the prism. Since the wavelengths of the seven colours are different, they come out separately from the glass prism.

A glass prism also refracts the light. The bending of the white mentioned above is known as the refraction of light. Red bends the least as it has the longest wavelength, whereas violet bends the most as it has the shortest wavelength. Because of varying wavelengths and bending of light, we see dispersion through a glass prism.

Prism experiment

The first prism experiment was conducted by Newton. It was an accidental experiment. He placed the prism near the place where sunshine came. He observed the sunlight was passing through one side of the prism, but different colours were coming out from the other side. Later he experimented to prove this concept. We can also use lenses to prove dispersion through chromatic aberration.

After confirming his experiment, he made several observations as:

1. He confirmed that white light splits into a spectrum of seven colours. But after he allowed a particular colour      to pass through the prism, he found that the results were not the same. The particular colour light underwent      refraction but did not undergo dispersion.
2. He then concluded that white light would only bend and disperse into seven colours. Violet will bend the           most as its wavelength is shortest, while red will bend the least as its wavelength is longest.

Why does light not bend while passing through a glass slab?
Strictly speaking, a glass slab also disperses the white light. But the effect is not the same as that of the prism. This is because the refractive indices of different materials are different. Therefore, their capacity to bend white light is also different. This happens because the frequency and wavelength of white light travel as different values in different refractive indices.

A glass slab has a higher refractive index than a prism (1.5 is greater than 1.4). Therefore, when the white light passes through a glass slab, it firstly moves slowly in the slab. Since the path is parallel inside the glass slab, the dispersion speed is relatively smaller than that of a prism. Therefore, we see a small dispersion when white light passes through a glass slab.

Whereas in the case of a prism, the bending inside the prism is fast, and the observer sees the spectrum instantaneously. The surface inside the prism is not parallel. It is made so that it forms different internal angles to allow the bending of light at a fast speed.

Uses of prism dispersion

1. Prism dispersion is used in performing laser tuning and spectroscopy.
2. Various prisms are made of different substrates so that they disperse only a particular wavelength of light. For example, the infrared wavelength will be separated through an infrared substrate prism. Infrared substrates include Zinc Selenide (ZnSe), Germanium (Ge), or Calcium Fluoride (CaF2).
3. Calcium Fluoride (CaF2) has a low refractive index which is ideal for ultraviolet through infrared.
4. Germanium (Ge) is mostly used to maximize optical paths due to its high refractive index.
5. Zinc Selenide (ZnSe) is used with CO2 lasers.
6. Dispersion is widely used in fibre optics. The material and waveguide dispersion effectively cancel each other. These produce a zero-dispersion wavelength. As a result, communication in fibre-optics is faster.

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