Dispersion of White Light by a Glass Prism: Principle and Spectrum Formation

Dispersion of white light by a glass prism is an optical phenomenon that shows how white light is actually made up of many colours. When a beam of sunlight passes through a glass prism, it separates into a beautiful band of colours known as a spectrum.

Imagine seeing a rainbow after a shower of rain. Have you ever wondered why it shows so many different colours? Interestingly, a glass prism creates a similar effect by separating white light into its component colours. This article explains the dispersion of white light by a glass prism, its cause, the role of a prism, and how the spectrum is formed.

Table of Contents

• What is a Prism
• What is Dispersion of Light
• Dispersion of White Light by a Glass Prism
• Why Do Different Colours Deviate Differently
• Applications of Dispersion of Light
• Frequently Asked Questions on Dispersion of White Light by a Glass Prism

What is a Prism

A prism is a transparent optical object made of glass or another transparent material. It usually has two triangular faces and three rectangular surfaces.

Its main function is to refract light. When light enters and leaves the prism, its direction changes due to refraction.

Prisms are commonly used in optical experiments to study the behaviour of light.

Important Terms Related to a Prism

Before moving further, let's look at some important terms associated with a prism.

Angle of Prism: The angle formed between the two refracting surfaces of a prism is called the angle of the prism or 'refracting angle'.

Angle of Deviation: The angle between the original path of the incident ray and the final path of the emergent ray is called the angle of deviation.

The relation between these angles is:

i₁ + i₂ = A + δ

Where:

• i₁ = Angle of incidence
• i₂ = Angle of emergence
• A = Angle of prism
• δ = Angle of deviation

What is Dispersion of Light

Dispersion is the splitting of white light into its constituent colours when it passes through a prism.

White light appears colourless, but it is actually made up of seven colours. When these colours pass through a prism, they bend by different amounts and separate from one another.

This separation of colours is called dispersion of light.

Dispersion of White Light by a Glass Prism

When a narrow beam of white light falls on a glass prism, it undergoes refraction at the first surface of the prism.

As the light enters the glass medium, its speed changes. Since each colour has a different wavelength, each colour travels slightly differently inside the prism.

As a result, the colours bend through different angles and separate from one another.

When the light emerges from the second surface of the prism, a band of seven colours is formed. This band of colours is known as the spectrum.

The prism itself does not create the colours. It only separates the colours already present in white light.

Formation of Spectrum

The spectrum formed by a prism consists of seven colours arranged in the following order:

Violet – Indigo – Blue – Green – Yellow – Orange – Red

This sequence is often remembered using the short form VIBGYOR.

Why Do Different Colours Deviate Differently

The fact is that different colours have different wavelengths. The amount of bending depends on the wavelength of the colour.

• Violet light has the shortest wavelength.
• Red light has the longest wavelength.

Since shorter wavelengths bend more, violet light undergoes the maximum deviation.

On the other hand, red light bends the least because it has the longest wavelength.

So, in simple words, the wavelength of light determines the amount of deviation inside the prism.

Colours Formed in the Spectrum

Here’s a closer look at the colours formed during the dispersion of white light by a glass prism:

Colour	Relative Wavelength	Amount of Deviation
Violet	Lowest	Maximum
Indigo	Very Low	Very High
Blue	Low	High
Green	Medium	Moderate
Yellow	Medium-High	Less
Orange	High	Low
Red	Highest	Minimum

This is why violet appears at one end of the spectrum while red appears at the other.

Applications of Dispersion of Light

The dispersion of white light by a glass prism has many practical applications.

Some common applications include:

• Studying the properties of light
• Spectroscopy and spectrum analysis
• Optical instruments
• Scientific research laboratories
• Identification of chemical elements
• Formation of natural rainbows

The rainbow seen in the sky is one of the best examples of dispersion in nature.

In this article, we learned that the dispersion of white light by a glass prism occurs when white light splits into seven colours due to different amounts of refraction. We also explored the role of a prism, the formation of the spectrum, and why different colours deviate by different angles.