Prism Dispersion

Dispersion In Prism

Do you know the light that appears to us as white when seen through naked eyes is actually a mixture of seven different colours of light? This was first told by newton after he performed an experiment with the glass prism.

When he made a ray of white light pass through a glass prism of triangular shape, the white light split into a band of seven different colours on the white screen placed in front of the glass prism. This led to the discovery of the fact that white light is actually a mixture of light of seven different colours. Further, we will study what is dispersion, what is the prism, the reason behind the dispersion of white light, what colours are mixed to form a white light and so on. So let's get started

Dispersion of Light By a Prism

Let’s start with a diagram of light dispersion in a prism, it will help us understand this phenomenon more clearly and precisely.

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In the above figure, we can clearly see a beam of white light is passed through a glass prism. On entering the prism the white light splits into a broad patch of seven colours on a screen placed in front of the prism. The splitting up of white light into seven different colours of the ray of light when passed through a prism is called dispersion of light. This clearly shows us that the white light is made up of seven different colours which get separated when this white light is passed through any transparent surface like a glass prism.

Now we should know about the colours that are mixed to form the white light.

The Seven Colours

The band of seven colours which is formed on a white screen when a beam of white light is passed through a transparent material like a glass prism is called a spectrum of white light. 

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As we can see in the figure above:

The Seven Colours in the Band Appearing on the White Screen are:

Red, Orange, Yellow, Green, Blue, Indigo and Violet and these seven colours are together denoted as a term called VIBGYOR where V denotes Violet I denotes Indigo B denotes Blue G denotes Green Y denotes Yellow O denotes Orange and R denotes Red.

So now we know about a lot of things about the dispersion of white light and it's time to know the reason behind the dispersion of white light by a glass prism

Dispersion of White Light By a Glass Prism

The basic reason behind the dispersion of white light into seven different colours is because all the seven rays of light of different colours travel at different speeds through the glass prism. The degree of refraction also can be said as the bending of each light ray depends upon their individual speed while passing through the prism. All the different colours of light ray travel at different speeds in glass and so the angle of bending or angle of refraction of each light ray is different. Out of the seven colours in the spectrum of white light red is the colour which has a maximum speed in the glass prism and hence it's the angle of deviation is the least Because of which red colour forms the upper part of the spectrum. On the other hand, violet is the colour in the spectrum of white light which has minimum speed in the glass prism hence it's the angle of deviation is the most. Because of which Violet colour forms the lowermost part of the spectrum. All seven colours differ in their frequencies. We should know that frequency is inversely proportional to the wavelength. Hence, the order of colours of the spectrum in order of increasing frequencies and decreasing wavelength are Red, Orange, Yellow, Green, Blue, Indigo, Violet. 

What is a Prism?

A prism is a triangular object made up of glass which is transparent. It has two triangular ends and three rectangular faces or three rectangular sides. The opposite faces of the glass prism are not parallel to each other but are opposite to each other. Also, these opposite faces of a triangular prism are inclined at an angle to one another. The angles between the opposite faces of the glass prism are called the angle of the prism.

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Above is the diagram of a prism. This is the real prism but in order to explain the phenomenon of dispersion and for the sake of ease of understanding the glass prism is considered to be a perfect triangle as shown in the figures above while explaining the dispersion of white light and formation of the spectrum.

Fun Fact

Dispersion of white light cannot only be seen through making a white light pass through a glass object like a prism. This phenomenon occurs even naturally. Have you ever seen a rainbow? The beautiful rainbow is the best natural example of dispersion of light. The rainbow is an arch of seven different colours visible in the sky produced because of the dispersion of sunlight through the raindrops.

FAQ (Frequently Asked Questions)

1. What do we Understand by the Dispersion of Light?

Dispersion is a phenomenon that happens when white light is made to pass through a transparent glass object like for example a glass prism. The splitting up of white light into seven different colours, when made to pass through a transparent object like a glass prism, is known as the dispersion of light.

This phenomenon takes place because the seven colours in the white light travel at different speeds when made to pass through a glass object.

2. What is the Composition of White Colour Light?

Ans. A white light is made up of seven different colours which can be seen clearly after the dispersion of white light when made to pass through a glass object like a prism.

The seven different colours that the white light is made up of are Violet, Indigo, Blue, Green, Yellow, Orange and Red 

While the red colour forms the upper part of the spectrum the violet colour forms the lower part of the spectrum.