Dispersion of Light By A Prism

Introduction

Refraction is a natural phenomenon of bending light from one medium to another. The refraction occurs because of the change of speed of wave light on entering into another medium. This refraction is the cause of various other physical phenomena such as dispersion. 

A prism is a well-polished optical element that is made to refract light. When the light is passed through this prism, it gets dispersed. Let us understand the whole concept in detail. 

What is Dispersion of Light? 

Dispersion is the phenomenon in which when light is passed through a transparent medium as a glass prism, it gets split into its constituent colours. 

The white light is made from seven colours, namely: 

• Violet
• Indigo
• Blue
• Green
• Yellow
• Orange
• Red

So on dispersion, the white light gets split into these seven colours. This band of seven colour components is said to be the  visible spectrum. 

The dispersion of light causes various natural phenomena. The most basic and important one is the formation of the rainbow. We will learn about it later in this article. Before that, let us understand why dispersion occurs?

Cause of Dispersion of Light

All seven colours have distinct wavelengths ranging from 380 nm to 740 nm. The red colour has the longest wavelength, whereas the violet has the shortest one. When all these seven colours move at the same speed and in the same phase, the visible white light is seen. 

We need to understand the important concept that refraction occurs due to a change in wavelength. When the light moves from rarer to a denser medium, its speed decreases and bends toward the normal. And in the opposite scenario that is on moving from denser medium to rarer, it bends away from normal. 

Now what happens is the white light is made of seven colours with distinct wavelengths and frequency. They move with the same speed and thus form visible white light in a vacuum. But as it enters the prism, refraction occurs. 

Since all seven colours have distinct wavelengths, they bend at different angles and move at different speeds. Due to this reason, when the light emerges from the other side of the prism, we get to see the spectrum of coloured bands. 

As the light has to undergo refraction two times on two different faces of the prism, therefore the spectrum thus formed is quite significant. 

Since violet has the least wavelength, it travels slowest, and thus it bends the most. Similarly, red is associated with the highest wavelength and travels the fastest. This leads to the least refractive index of red colour, and thus it bends the least. 

The dispersion of light by a prism is shown in the figure below: 

Newton’s prism experiment

Till 1660 people used to believe that the combination of light and darkness formed colours and that the prism used to colour the light; Hooke put forward this theory. But in the late 1660s, Newton started experimenting with light to prove Hooke’s theory false. 

He completely darkened his room one fine sunny day and set up a prism near his window. The dispersion of light occurs, producing a well-defined spectrum of coloured bands. Now to prove that it was not the prism that coloured the light, he put an inverted prism aligned with the first prism.  

He observed that the seven colours combined to on passing through an inverted prism and form the white light. Thus this proved that the prism was not colouring the light, and the seven colours were the property of the white light. That is, the combination makes white light of white light. 

Examples of dispersion of light

There are various occurrences based on the dispersion of light by a prism. Some of these examples are as follow: 

• A rainbow is a natural occurrence that is caused by the dispersion of sunlight by the prism as a rainbow
• The lubricants or filling such as oil used on roads act as a prism when it rains, and coloured bands are formed
• When the white light passes through the diamond, it gets dispersed and reaches the viewer’s eye as sparkling colours known as diamond fire
• Soap bubbles act as the prism, and when light falls, it gets diffracted to form beautiful colours
•  When a compact disk is faced towards the light, it shows various colours caused by the dispersion of light by a prism. Here a compact disk acts as a prism

Formation of rainbow 

The rainbow is a meteorological phenomenon that is a general example of the dispersion of light by a prism. The sunlight is made of seven colours that move at the same speed and is seen as visible white light; when it rains, the rain droplets in the atmosphere act as small prisms. 

When passed through raindrops, the sunlight forms various coloured spectrum bands at 2 degrees angles. The light gets refracted twice on passing through the rain droplets. The spectrum thus formed is not a true spectrum and thus appears blurry. 

The observer must be positioned specifically relative to the sun to see the rainbow thus formed. The observer should stand in such a way that the sun should be behind the observer. The rainbow is formed as a complete circle. But from the ground, we are able to see some portion of it as an arc.

Conclusion 

The refraction of light causes diffraction of light by a prism passing through a prism. The dispersion of light causes the splitting of light into seven colours on passing through a prism. All the colours of light have distinct wavelengths and move at the same speed in the vacuum or normally. On passing through any medium, refraction occurs, and the speed of the colours changes; therefore, the spectrum is formed. Many natural phenomena can be explained through the dispersion of light by a prism.