[Physics Class Notes] on Refraction and Dispersion of Light Pdf for Exam

Refraction is the bending of light as it passes through a transparent substance. Water, sound, and other waves do it too. This bending had allowed the creation of magnifying glasses, prisms, lenses, etc. 

Here, the degree to which refraction occurs relies on the light’s wavelength. Each light wave has a range or set of wavelengths and will so deviate in a different direction. 

Key Features of Refraction

• Refraction is important in the lens, eye, sound, water, and focal length formation. 

• In a slower medium, the wavelength is also shortened.

• The Index of Refraction describes how light in a medium is divided by light in a vacuum. The formula is n=c/v, where n is the index of refraction, c the vacuum velocity, and v the medium velocity.

Types of Refraction 

Diffuse refraction of light – It scatters light in a variety of directions.

Specular refraction – The angle at which light strikes a specular surface is the same as the angle at which the light strikes the surface.

Glossy refraction – A glossy surface has micro-surfaces angled to the surface plane.

Light is refracted when:

• No change in frequency of the refracted beam.

• Partially reflected and absorbed light at the contact reduces the strength of refracted rays.

• Light deviates when it crosses a border between two mediums. Light’s wavelength and speed vary during refraction.

Refractive Index

The refractive index of the material medium is the ratio of the speed of light in a vacuum to the speed of light in the material medium.

How much of a wave has been refracted is determined by the difference in speed and the initial direction of propagation relative to the direction of speed change.

Dispersion of Light

When light passes through a transparent medium, dispersion is defined as the splitting of the light beam into its seven constituent colours.

Sir Isaac Newton described this occurrence in 1666 A.D. When sunlight passes through a glass prism, he discovered that white light is made up of seven distinct hues.

A rainbow against a dark stormy sky is a sight to behold. How does sunlight on clean raindrops create the rainbow of colours we see? A clear glass prism or a diamond employ the same method to colourize white light.

What causes this to happen? This is due to a phenomenon known as ‘Dispersion of Light,’ which occurs in conjunction with refraction.

The spreading of white light into its complete spectrum of wavelengths is known as dispersion. The spectrum of colours are:

• Violet

• Indigo

• Blue

• Green

• Yellow

• Orange 

• Red

“Spectrum” refers to a ring of brightly coloured lights.

An illustration of light dispersion via a glass prism can help comprehend it better.

Glass Prism – Dispersion of Light

The prism is a five-sided solid with two triangle bases and three rectangular surfaces that are angled inwards.

One of the rectangular faces sends light into the prism, which enters through one of the other rectangular faces and exits through the other rectangle face. The refractive index of different hues of light varies because they travel at different speeds. As a result, when white light passes through the prism’s refracting surface, its constituents bend at different angles, splitting the single beam of light. Because of the refraction induced by the second rectangular surface, the distinct colours of light bend again.

When white light passes through a glass prism, it is split into its component colours. The sole reason for this is Refraction.

After polychromatic light enters from a less dense medium to a large dense medium, refraction causes each colour of light to take a separate path.

Causes of Dispersion of Light 

The cause of dispersion of light through the prism is that white light has a range array of seven colours, and each of those has a subsequent angle of deviation. As such, when light passes through a prism, different colours deviate from different angles. Therefore, those colours get separated and form a series of bands called a spectrum. Out of those seven colours, the red one deviates the least and has got the position on top of the spectrum. Whereas the violet colour deviates most, that is why it has got the position at the bottom of the spectrum.

Here, the sole cause of dispersion of light is refraction.

Because of refraction, every colour of light takes a different path after polychromatic light enters from the less dense medium to a large dense medium. This happens as per Snell’s law. It states that sin()/sin(r) is different for a different colour of light and medium where it travels. Therefore, the split light represents the component of the original incident light.

The above-mentioned explanation shows how dispersion occurs. One thing to be noted here is that in the case of normal incidence, dispersion and refraction doesn’t occur.

Fun facts- Have you ever seen the rainbow and got mesmerized by its natural beauty? They are the perfect phenomenon that occurs and is the best example to bring light for “dispersion of light” alongside refraction. This is the reason you can see rainbow-like occurrences in both crystals and prisms. 

What is Dispersion of White Light?

Dispersion of White Light by a Prism is shown here below:

Wavelength is inversely proportional to the deviation where the light travels. Here, the prism only acts as a medium for dispersion made of seven different colours. Further, refraction occurs when light rays fall on it, and depending on that, the frequency and wavelength deviate differently at a different angle because of the difference in their velocities. The colour deviates the least because it has a maximum wavelength, and the violet colour deviates the most because of its lesser wavelength.

The reason for light dispersion through prisms is because white light has a range of seven hues, each with its angle of deviation.

Light passing through a prism deviates from one colour to another. So the colours divide into a spectrum of bands. 

Red is the colour that deviates the least from the others and is, therefore, the most dominant. 

Light dispersion is caused only by refraction.

After polychromatic light enters a big thick medium, refraction causes each colour to take a separate route. Snell’s law dictates this. It states that sin(i)/sin(r) varies with light colour and medium. So the split light is the original incident light component.

Types of Dispersion

There are several types of dispersion, each of which functions in a unique way, but the three most common are detailed below:

1. Material dispersion (chromatic dispersion)

Rather than a single narrow wavelength, both lasers and LEDs create a variety of optical wavelengths (a band of light).

At different wavelengths, the fibre has varied refractive index characteristics, hence each wavelength travels at a different speed in the fibre.

As a result, some wavelengths arrive ahead of others, causing a signal pulse to disperse (or smears out).

2. Mode dispersion (intermodal dispersion):

When light travels through a multimode fibre, it can take many different routes or “modes” as it travels through the fibre.

Each mode’s distance travelled by light differs from the distance travelled by other modes.

Parts of a pulse (rays or quanta) can adopt several distinct modes when it is transmitted (usually all available modes).

As a result, some pulse components will arrive before others. As the distance between the fastest and slowest modes of light increases, the difference in their arrival times increases.

3. Dispersion of the waveguide

The form and index profile of the fibre core generate waveguide dispersion, which is a very complex phenomenon. However, the proper design can manage this, and waveguide dispersion can even be utilised to counteract material dispersion.

Different fibres’ dispersion:

Waveguide dispersion > mode dispersion > material dispersion