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Refraction and Dispersion of Light

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Last updated date: 17th Apr 2024
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About Refraction of Light

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:


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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).

  1. 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.

  1. 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

FAQs on Refraction and Dispersion of Light

1. Explain why Planets don't Twinkle but the Star does.

Stars twinkle because of the refraction of starlight in the atmospheric region. When the light from the star enters the atmosphere of the earth, it undergoes continuous refraction before it reaches the earth's surface. The atmospheric refraction is because of the change in the refractive index at a range of atmospheric levels. Here, the starlight bends towards the normal, where the apparent point of stars is different from that of the actual one.


Since the atmospheric region doesn't remain constant, it changes. Since the travelling orbit of light rays coming from the star varies slightly, then the virtual position also varies and so the amount of light entering our eyes does flicker. Sometimes, it remains bright and sometimes blurs. Because of the same reason, stars twinkle and planets do not as they are close to the earth and are said to be extended sources. If a planet is taken as a collection of point sources, then the total light that enters our eye on average is zero. This makes the twinkling effect of the planet to be null and void.

2. Give some examples of Dispersion of Light.

Given below is the dispersion of light examples

  1. Dewdrops in the morning during winter

  2. Formation of rainbow

  3. The Compact Disc

  4. Diamond

3. Explain the Formation of the Rainbow.

We can explain the formation of the rainbow by considering the process of dispersion of light. When it comes to the rainbow, we see its seven colours in the sky just after the rain when the sun shines. Here, the crucial condition to see a rainbow is that one should stand with his back facing the sun to see the rainbow. 


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Normally, after rainfall, a huge number of water droplets get suspended in the atmosphere. These droplets of water function as small prisms. Therefore, when the white light emitted by the sun falls on the droplets, then the light gets split into several different colours and the rainbow gets formed.

4. What is the Difference Between Dispersion and Refraction of Light?

The main difference between refraction and dispersion of light is given here below:


As mentioned earlier, refraction is any bending of waves due to the change in speed. When water waves move through a range of depths and heights, there is the formation of refraction. 


On the other hand, dispersion depends on the frequency and when we say that light is refracted by a prism, dispersion here means there will be a more high frequency of bending of light.


As such, refraction is the bending of light, and dispersion is the frequency dependency of the characteristics of light.

5. What does the Law of Refraction state?

Since we've gone over the basics of refraction, it is also important to know the two most significant laws of refraction-

  1. The normal interface of the two media at the point of incidence, the reflected ray, and the incident ray all lie on the same plane.

  2. The angle of refraction's sine to the angle of incidence's sine is a constant. Snell's law of refraction is also known as this.

\[\frac {Sin i} {Sin r}\] = Constant 

6. Why does light in a prism split into its colour spectrum but not in a glass slab?

Light disperses into its rainbow of colours in a glass slab. We can see this if we look at it carefully. Before we begin, you should understand refractive indices. They change. They change with light frequency and thus wavelength.


White light refracted twice to pass through a glass slab or a prism. It moves from glass to air, then back to glass. It slows down at first, then speeds up at second.


So, in a glass slab, Because all surfaces are parallel, all light beams slow down and speed up uniformly. So it appears to an onlooker that white light has entered and exited the slab. But in a prism, it's different.


Because the surfaces aren't parallel, the light rays exiting the prism take a varied path, resulting in a dispersed effect.

7. What are the different applications of Refraction of Light?

It has numerous uses in both nature and human life. The refraction of light is responsible for the development of rainbows and mirages in nature. 


The refraction of light plays a crucial part in human life. It has a wide range of uses in optics and imaging. The following are some of the applications:

  • Refraction To correct refractive defects in humans, concave and convex glasses are utilized.

  • The principle of refraction of light is used in peepholes indoors, magnifying glasses, binoculars, cameras, projector lenses, and other devices.

8. What are the elements that influence light refraction?

The following are some of the factors that influence light refraction:


Medium-density: When the medium's density is low, it does not refract light very well. The denser medium refracts light by slowing the speed of light, resulting in refraction.


Temperature and pressure: The variation in the refractive index of air at higher and lower altitudes explains the influence of temperature and pressure. 


The density of air is low at high altitudes, where the temperature and pressure are low. Hence, it refracts light much less than at low altitudes, when the pressure is high and the temperature is moderate.


Wavelength: There are seven primary colours in light. The wavelengths of these colours are different. When light flows through the medium, colours with shorter wavelengths refract less, while colours with longer wavelengths refract more.

9. What are the key differences between Reflection and Refraction?

Reflection

  • This is a common occurrence in mirrors.

  • The reflection of light as it strikes a medium on a plane is simply characterized as reflection.

  • The light that enters the medium is reflected in the same direction.

  • Light waves reorient themselves after bouncing.

  • The light's angle of incidence equals its angle of reflection.

Refraction

  • This is a common occurrence in Lenses.

  • Refraction is the process of shifting light across a medium, resulting in light bending.

  • Light entering the medium travels between mediums.

  • Light waves flow through the surface while changing mediums.

  • The incidence angle is not the reflection angle.

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