experiment of dispersion of light through prism

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Dispersion of Light through a Prism

Dispersion of Light happens when white light is split into its constituent hues due to refraction. Dispersion of Light can be achieved through various means but the most common way to achieve dispersion of light is through Prism. Dispersion of light by a prism results in the breaking of white light into its seven constituents.

Dispersion of Light through a prism is achieved by allowing the white light to fall on the prism and passing the light through the prism to break it into its constituent colours. In this article, we will learn about the Dispersion of Light, its experiment and others in detail.

What is Dispersion of Light?

Dispersion is defined as the spitting of white light into different colors when passed through a prism. 

The white light after passing through the prism splits into seven different colours namely, 

Together these colours are written as VIBGYOR.

Learn more about, Dispersion of Light .

Dispersion Of Light Through Prism

When light passes from one medium to another medium speed of propagation of light changes as a result the light is refracted. Now when the light passes through the prism, it gets refracted this refraction of light makes the light split into various colours and this phenomenon is called the dispersion of light through the prism.

Different colours in the light range have different wavelengths. Therefore, the speed at which they bend varies depending on the wavelength, in which the Violet light bends most, and the Red bends the least. As a result, white light coming out from the prism breaks into the spectrum of the light.

Diagram of Dispersion of Light Through Prism

The image below shows the dispersion of light through a prism and the formation of a spectrum by white light.

Angle of Deviation

The measure of refraction in the path of light after passing through the prism is measured by measuring the angle of deviation. The angle of deviation is defined as the angle made between the incident ray of light entering the prism and the refracted ray emerging out of the prism.

The deviation of the light waves after passing through the prism is inversely proportional to the wavelength of the light as the wavelength of the Violet light is the least and so the violet light get deviates the most, whereas the wavelength of the red light is the most and hence it gets deviates the least.

The image below shows the angle of deviation:

The visible pattern of the spectrum observed by us when the light passes through the prism is because of the change in the wavelength of the various colour lights.

Visible Light Spectrum

The light disperses into a wide range of colours after passing through a glass prism. We can see this by looking at it from a different perspective. The refractive index associative with a material is not fixed it varies with the frequency of the light used.

• If white light passes a glass prism it gets refracted twice, first when the light strikes the prism the light ray gets deviated from the air to the glass surface and its speed decreases whereas when the light ray leaves the glass prism it again gets deviated from the glass to the air surface and its speed is increased.
• Inside the glass prism, the speed of the light rays remains constant.
• The surface of the prism is not parallel and hence the light ray does not follow the same path and gets deviated.

This deviation in the white lights makes the spectrum of the light visible and we observe the visible spectrum of the light. For example, the spectrum observed on the oil drop is the visible spectrum of the light.

Prism Experiment

The first experiment of light passing through the prism was first conducted by the great scientist Newton. He allows white light to pass through a prism hoping to get white light to the other end. But to his surprise, he found that white light gets changed to the spectrum of the seven colours. He named this phenomenon as dispersion of light.

Through this experiment, he concluded that light is made up of a spectrum of light. To further prove his experiment he from some other tests as

• He allows the light of only one wavelength to pass through the prism and observes the light ray coming out of the prism and found that this ray only gets refracted and so no sign of dispersion.
• He realized that only white light shows dispersion, as it is made of several colours of light and all these colours of light, have different wavelengths thus their refraction is not the same they all get deviate differently passing through the prism and allowing to form the spectrum of light. 
• He concludes that violet light has the shortest wavelength and hence it deviates most whereas red light has the highest wavelength and hence it deviates the least.

Examples of Dispersion of Light

Various examples where the dispersion of light is observed are,

Formation of Rainbow 

Dispersion of light is the reason behind the formation of the rainbow. When it rains the tiny water droplets remain in the air. When the sunlight passes through water droplets the light gets dispersed and we see the dispersed light in the form of a Rainbow.

Spectrum observed on Oil Droplets in Water

When oil droplets fall on water, we see the different colours in them. This is because the light undergoes refraction when it passes from the oil to the water medium or vice versa. Hence the light is dispersed and we see the spectrum of different colours in the oil droplets.

Reflection of Light Scattering of Light Polarization of Light

FAQs on Dispersion of Light

Q1: what is dispersion of light.

When white light passes through a glass prism, it separates into its spectrum of colours (in order violet, indigo, blue, green, yellow, orange, and red), this process is known as Dispersion.

Q2: What is Prism?

A prism is glass apparatus made of transparent glass. It has three rectangular lateral surfaces and two triangular faces that are inclined at an angle. It dispersed the white light passing through it.

Q3:What is Refraction of Light?

When the light ray changes its medium of propagation its deviates from its path this phenomenon is called the refraction of the light.

Q4: What is the Difference between Reflection and Refraction of Light?

Refraction of light is the change in the direction of light when it changes its propagation of the medium whereas reflection is the change in direction of the light after striking through a solid surface.

Q5: What are Examples of Refraction of Light?

Various examples of the refraction of light are, Twinkling of Stars Formation of Rainbow Red light of the sky during sunrise and sunset

Q6: What is VIBGYOR Full Form?

When the light ray is passed through the glass prism it gets deviated into the spectrum of the light in the order VIBGYOR and the full form of the VIBGYOR is, V iolet I ndigo B lue G reen Y ellow O range R ed

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• Physics Article
• Tracing The Path Of The Rays Of Light Through A Glass Prism

Tracing the Path of the Rays of Light Through a Glass Prism

A prism is a transparent optical object with two flat surfaces that have an angle between them. When the light enters the prism, there is a bending of light as there is a change in the speed of light. The bending of the light is dependent on the angle of incidence, normal, and refractive indices. There are four different types of angles involved in this experiment, and they are the angle of incidence, angle of emergence, angle of prism and angle of deviation. Below is an experiment to trace the path of a light ray through a glass prism.

To trace the path of the rays of light through a glass prism.

What Is a prism?

A prism is defined as a polyhedron with a triangular base and three rectangular lateral surfaces. It is used as an optical object to study the behaviour of white light when it is passed through it. The light bends at various angles like an angle of incidence, angle of reflection, angle of refraction, and angle of deviation.

Read More: Angle of Incidence

What Is the prism formula?

Following is the formula of the angle of prism:

where, µ is the refractive index. A is the angle of the prism. δ m is the minimum deviation.

What Is the angle of deviation?

The angle of deviation is defined as the angle between the incident ray and the emerging ray.

Materials Required

Following are the list of materials required for this experiment:

• A white sheet
• 4-6 all pins
• Drawing board

Experimental Setup

• Fix a white sheet on a drawing board using drawing pins.
• Place the triangular prism resting on its triangular base. Using a pencil, draw the outline of the prism.
• Draw NEN normal to the face of the prism AB. Make an angle between 30 ° and 60 ° with the normal.
• On the line PE, fix two pins at a distance of 5cm from each other and mark these as P and Q.
• Look for the images of the pins at P and Q through the other face of the prism AC.
• Fix two pins at R and S such that they appear as a straight line as that of the P and Q when it is viewed from the AC face of the prism.
• Remove the pins and the prism.
• At point F, make the points R and S meet by extending them.
• PQE is the incident ray which is extended till it meets face AC. SRF is the emergent ray which is extended backward to meet at point G.
• Now mark the angle of incidence ∠i, angle of refraction ∠r and the angle of emergence ∠e and the angle of deviation ∠D as shown in the experimental setup.
• Repeat the experiment for more angles between 30 ° and 60 °.

Observations

• At surface AB, the light ray enters and bends towards the normal on refraction.
• At surface AC, the light ray bends away from the normal as it travels from one medium (glass) to the other (air).
• The angle of deviation is observed. Here, the emergent ray bends at an angle from the direction of the incident ray.
• The incident ray bends towards the normal when it enters the prism and while leaving the prism it bends away from the normal.
• With the increase in the angle of incidence, the angle of deviation decreases. After attaining the minimum value, it increases with an increase in the angle of incidence.

Precautions

• For drawing the boundary of the prism, a sharp pencil should be used.
• Soft board and pointed pins should be used.
• The distance between the pins should be 5cm or more.
• The pins should be fixed vertically and should be encircled when they are removed from the board.
• The angle of incidence should be between 30 ° and 60 ° .
• The arrows drawn for incident ray, reflected ray and emergent ray should be proper.
• For viewing the col-linearity of all four pins and images, the head should be slightly tilted on either side. While doing this it can appear as if all are moving together.

Read More: Refractive Index

Q1. What is refractive index? Ans: Refractive index is defined as the ratio between the sine of the angle of incident ray i in a vacuum to the sine of the angle of refraction r in a given medium. The mathematical representation of the refractive index is given as:

• n is the refractive index
• i is the angle of the incidence
• r is the angle of the reflection

Q2. What is the unit of the refractive index? Ans: Refractive index has no unit. This is because it is a ratio between the speed of light in vacuum and in a medium.

Q3. Name the atmospheric refraction that causes the splitting of white light. Ans: Rainbow.

Q4. What is the dispersion of light? Ans: The dispersion of light is defined as the phenomenon of splitting white light into its seven constituent colours when it is made to pass through a transparent medium.

Q5. List the factors on which the angle of deviation through a prism depends. Ans: Following are the factors on which the angle of deviation through a prism depends:

• ∠A is called the angle of a prism
• Angle of incidence
• On the optical density of the material used in a prism.

Q6. What happens to the incident ray when it enters the prism? Ans: When an incident ray enters the prism, it bends towards the normal as it changes its path and gets deviated.

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• Dispersion of Light by Prisms
• Rainbow Formation

In the Light and Color unit of The Physics Classroom Tutorial, the visible light spectrum was introduced and discussed. Visible light, also known as white light, consists of a collection of component colors. These colors are often observed as light passes through a triangular prism. Upon passage through the prism, the white light is separated into its component colors - red, orange, yellow, green, blue and violet. The separation of visible light into its different colors is known as dispersion . It was mentioned in the Light and Color unit that each color is characteristic of a distinct wave frequency; and different frequencies of light waves will bend varying amounts upon passage through a prism. In this unit, we will investigate the dispersion of light in more detail, pondering the reasons why different frequencies of light bend or refract different amounts when passing through the prism.

Earlier in this unit, the concept of optical density was introduced. Different materials are distinguished from each other by their different optical densities. The optical density is simply a measure of the tendency of a material to slow down light as it travels through it. As mentioned earlier, a light wave traveling through a transparent material interacts with the atoms of that material. When a light wave impinges upon an atom of the material, it is absorbed by that atom. The absorbed energy causes the electrons in the atom to vibrate. If the frequency of the light wave does not match the resonance frequency of the vibrating electrons, then the light will be reemitted by the atom at the same frequency at which it impinged upon it. The light wave then travels through the interatomic vacuum towards the next atom of the material. Once it impinges upon the next atom, the process of absorption and re-emission is repeated.  

The optical density of a material is the result of the tendency of the atoms of a material to maintain the absorbed energy of the light wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. Thus, while a light wave travels through a vacuum at a speed of c (3.00 x 10 8 m/s), it travels through a transparent material at speeds less than c . The index of refraction value ( n ) provides a quantitative expression of the optical density of a given medium. Materials with higher index of refraction values have a tendency to hold onto the absorbed light energy for greater lengths of time before reemitting it to the interatomic void. The more closely that the frequency of the light wave matches the resonant frequency of the electrons of the atoms of a material, the greater the optical density and the greater the index of refraction. A light wave would be slowed down to a greater extent when passing through such a material

The Angle of Deviation

Of course the discussion of the dispersion of light by triangular prisms begs the following question: Why doesn't a square or rectangular prism cause the dispersion of a narrow beam of white light? The short answer is that it does. The long answer is provided in the following discussion and illustrated by the diagram below.

Suppose that a flashlight could be covered with black paper with a slit across it so as to create a beam of white light. And suppose that the beam of white light with its component colors unseparated were directed at an angle towards the surface of a rectangular glass prism. As would be expected, the light would refract towards the normal upon entering the glass and away from the normal upon exiting the glass. But since the violet light has a shorter wavelength, it would refract more than the longer wavelength red light. The refraction of light at the entry location into the rectangular glass prism would cause a little separation of the white light. However, upon exiting the glass prism, the refraction takes place in the opposite direction. The light refracts away from the normal, with the violet light bending a bit more than the red light. Unlike the passage through the triangular prism with non-parallel sides, there is no overall angle of deviation for the various colors of white light. Both the red and the violet components of light are traveling in the same direction as they were traveling before entry into the prism. There is however a thin red fringe present on one end of the beam and thin violet fringe present on the opposite side of the beam. This fringe is evidence of dispersion. Because there is a different angle of deviation of the various components of white light after transmission across the first boundary, the violet is separated ever so slightly from the red. Upon transmission across the second boundary, the direction of refraction is reversed; yet because the violet light has traveled further downward when passing through the rectangle it is the primary color present in the lower edge of the beam. The same can be said for red light on the upper edge of the beam.

Dispersion of light provides evidence for the existence of a spectrum of wavelengths present in visible light. It is also the basis for understanding the formation of rainbows. Rainbow formation is the next topic of discussion in Lesson 4.

• The Anatomy of a Lens

'Refraction is then all there is to it': How Isaac Newton's experiments revealed the mystery of light

"The colors of the spectrum, then, "are not Qualifications [alterations] of Light … (as 'tis generally believed), but Original and connate properties."

The beauty and majesty of rainbows have inspired awe in humans for millennia, but it wasn't until Isaac Newton's groundbreaking work unlocking the secrets of light did we truly begin to understand how they form.

In this extract from the new book " Beautiful Experiments: An Illustrated History of Experimental Science " (The University of Chicago Press, 2023), science writer Philip Ball explains how Isaac Newton's ingenious experiment with prisms transformed our understanding of light.

The puzzle of the rainbow was resolved in the seventeenth century through the work of the scientist who some regard as the greatest ever to have lived. In 1666, Isaac Newton — then a 23-year-old Cambridge graduate — performed an experiment with light that transformed our understanding of it. 

While it was thought that the bar of rainbow colors — called a spectrum — produced when white light (like sunlight) travels through a glass prism is caused by some property of the prism that alters the light, Newton showed the colors are already inherent in the light itself. Legend has it that Newton did the experiment at his family home in Woolsthorpe, Lincolnshire, to which he had returned to escape the Great Plague that ravaged England in 1665. 

It did not, after all, require any fancy apparatus — just a few prisms, which could be bought almost as trinkets at markets (although he needed good-quality ones!). While there's truth in that, Newton had been planning such experiments for a while in his Cambridge room: we need not credit the plague for stimulating this leap in understanding optics. Newton didn't report his results until six years later, when he sent an account to the Royal Society in London, the intellectual center of "experimental philosophy" in the mid-century. 

Related: 9 equations that changed the world

He was famously reluctant to disclose the outcomes of his studies, and had to be cajoled into writing down his celebrated laws of motion and theories about the motions of the planets in his masterwork the Principia Mathematica in 1687. The book in which he recorded his experiments and theories about light, Opticks, was finally published in 1704. This was not so much because Newton was diffident about his work; on the contrary, he was rather covetous about it, and highly sensitive to criticism. 

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Newton begins his 1672 account by relating his surprise that the colored spectrum produced by his prism was rectangular in shape rather than circular, "as the received laws of Refraction" would lead one to expect. It seems a rather trifling question, especially to lead to such profound conclusions. In fact, his "surprise" is hard to credit, for this effect of a prism was well known, not least to Newton himself, who had been fascinated with such instruments since he was a boy. 

Newton was here no doubt indulging what is now a common practice in scientific papers: to construct a retrospective story so as to give a comprehensible narrative arc to a description of experiments that might have a more haphazard genesis and perhaps initially a different goal entirely. At any rate, Newton embarked on a thorough program of experimentation to figure out what the prism was doing to light. 

One can imagine him almost literally playing with prisms, screens, and lenses until he found a configuration that allowed him to formulate and investigate some definite hypotheses. (Newton once famously claimed that "I feign no hypotheses," but in truth one can hardly do science at all without them.) 

But only Newton saw what this implies: that refraction is then all there is to it

It's a common situation for experimental science: you might want to investigate a phenomenon but be unsure quite what the right questions are, let alone how to deploy your instruments and measuring devices to answer them. You need to develop a feeling for the system you're trying to study. 

Newton closed the "window-shuts" of his room, admitting a single narrow beam of sunlight through a hole, which passed into the prism. In the crucial experiment, Newton investigated the nature of the light after it exited the prism. If the light became colored because of some transformation produced by the prism, then a passage through a second prism might be expected to alter the light again. 

Newton used a board with a hole in it to screen off all the spectrum except for a single color — red, say — and then allowed that colored light to pass through the second prism. He found that this light emerged from the second prism refracted — bent at an angle — but otherwise unchanged. In other words, a prism seems only to bend (refract) light, leaving it otherwise unaltered. But it does so to different degrees (that is, at different angles) for different colors. 

This in itself was nothing new: the Anglo-Irish scientist Robert Boyle had said as much in his 1664 book "Experiments and Considerations Touching Colours," which Newton had read. But only Newton saw what this implies: that refraction is then all there is to it. 

The colors themselves are already in the white light, and all the prism does is to separate them out. As he put it, "Light consists of Rays differently refrangible" [meaning refractable]. The colors of the spectrum, then, "are not Qualifications [alterations] of Light … (as 'tis generally believed), but Original and connate properties." That was a bold interpretation: sunlight was not, so to speak, elemental, but compound. 

To test this idea, Newton used a lens to refocus a many-hued spectrum into a single, merged beam — which, he observed, was white. He also passed this reconstituted beam through another prism to reveal that it could again be split into a spectrum just as before. 

Newton explained how his observations could account for the rainbow, produced by the refraction and reflection of light through raindrops that act as tiny prisms. The colors of everyday objects, he added, arise because they reflect "one sort of light in greater plenty than another." 

— What is visible light?

— Are rainbows really arches?

— 20 inventions that changed the world

And the results explained the defects of lenses (Newton himself had become adept at making these by grinding glass), whereby refraction of different colors produces a defocusing effect called chromatic aberration. The Royal Society's secretary Henry Oldenburg told Newton that his report was met with "uncommon applause" when read at a gathering in February 1672. But not everyone appreciated it. 

After the paper was published in the society's Philosophical Transactions, its in-house curator of experiments, Robert Hooke , who considered himself an expert on optics, presented several criticisms (which we can now see were mistaken). Newton replied with lofty condescension, igniting a long-standing feud between the two men. 

One problem is that Newton's experiments, despite their apparent simplicity, are not easy to replicate: some, in England and abroad, tried and failed. But they have stood the test of time, a testament to the power of experiment to literally illuminate the unknown that, in the judgment of philosopher of science Robert Crease, gives Newton's so-called experimentum crucis "a kind of moral beauty."

Reprinted with permission from Beautiful Experiments: An Illustrated History of Experimental Science by Philip Ball, published by The University of Chicago Press. © 2023 by Quarto Publishing plc. All rights reserved.

Beautiful Experiments: An Illustrated History of Experimental Science - $25.82 on Amazon

Philip Ball's illustrated history of experimental science is a celebration of the ingenuity that scientists and natural philosophers have used throughout the ages to study — and to change — the world.

If you enjoyed this extract you can read another extract from the book: How 18th century scientists figured out fertilization

Philip Ball is a freelance writer and broadcaster, and was an editor at Nature for more than twenty years. He writes regularly in the scientific and popular media and has written many books on the interactions of the sciences, the arts, and wider culture, including "H2O: A Biography of Water "  and "The Music Instinct. "  His book "Critical Mass "  won the 2005 Aventis Prize for Science Books. Ball is also the 2022 recipient of the Royal Society’s Wilkins-Bernal-Medawar Medal for contributions to the history, philosophy, or social roles of science. He trained as a chemist at the University of Oxford and as a physicist at the University of Bristol, and he was an editor at  Nature  for more than twenty years. 

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Dispersion Of Light Through A Prism

The story of light dispersion through a prism begins with the foundational work of Sir Isaac Newton in the 17th century. Newton was fascinated by the nature of light and its properties. His curiosity led to a series of experiments that would forever change our understanding of optics.

In 1666 , Newton conducted an experiment that was simple yet revolutionary. He allowed a beam of sunlight to pass through a triangular glass prism in a darkened room. To his surprise, instead of white light emerging from the other side, he observed a spectrum of colors.

Newton deduced that white light was not pure but a mixture of different colors. Each color had a different wavelength and bent by a different amount when passing through the prism. This bending, or refraction, caused the white light to spread out into a spectrum of colors, a phenomenon he named the dispersion of light .

Newton was meticulous and cautious about sharing his findings. It wasn’t until 1704 , after much persuasion, that he published his work in a book titled “Opticks”. In it, he detailed his experiments and theories on light, including the concept of dispersion. Newton’s work laid the groundwork for modern optics. His prism experiment was a pivotal moment in science, leading to the development of new optical instruments and enhancing our understanding of the nature of light.

Today, Newton’s insights into the dispersion of light continue to influence various fields, from spectroscopy to astronomy. His principles are taught worldwide, forming a fundamental part of physics education.

Table of Contents

What is the Dispersion of Light?

Dispersion of light is a fascinating and colorful phenomenon that occurs when white light is separated into its constituent colors. Dispersion of light is the separation of white light into its constituent colors when it passes through a medium like a prism. This occurs because different colors of light bend by different amounts due to their varying wavelengths.

Imagine white light as a team of runners, each wearing a different color shirt, racing through a medium like glass or water. As they run, they encounter a hurdle—the prism—which slows them down. But here’s the catch: each runner is slowed down by a different amount because of their unique shirt color, which represents the light’s wavelength.

In more scientific terms, dispersion happens because light is made up of waves, and these waves have different lengths. When white light enters a prism, each color of light is refracted, or bent, to a different degree. Violet light, with the shortest wavelength, is bent the most, and red light, with the longest wavelength, is bent the least. This separation of colors is what we call dispersion.

It’s like a musical band where each instrument plays a different note, and when they enter the prism, each note is directed to a different part of the room. The result is not just a single melody but a spectrum of musical notes spread across the space, similar to how light spreads across a spectrum of colors.

So, when we talk about the dispersion of light, we’re referring to this process of separating white light into a rainbow of colors. It’s a fundamental concept that explains why we see the colors we do and is essential for understanding the behavior of light as it travels through different mediums.

Dispersion of White Light by Glass Prism

The dispersion of white light by a glass prism occurs because different colors of light are refracted by different amounts due to their wavelengths. This results in the spread of white light into a continuous spectrum of colors, which we can observe as it exits the prism. When white light enters a glass prism, it refracts and splits into a spectrum of colors—red, orange, yellow, green, blue, indigo, and violet. Red light bends the least, while violet bends the most.

Imagine you have a beam of white light, like sunlight. This light contains all the colors of the rainbow, but you can’t see them because they’re all mixed. Now, let’s say you pass this white light through a triangular glass prism. What happens inside the prism is quite remarkable and is the essence of dispersion.

As the white light enters the prism, it encounters a change in medium from air to glass. This causes the light to slow down and bend—a process known as refraction. However, not all colors of light bend the same amount. Violet light, with its shorter wavelength, bends the most, and red light, with its longer wavelength, bends the least. This difference in bending causes the white light to spread out into a spectrum of colors.

Inside the prism, each color of light travels at a different speed because the glass prism has a different refractive index for each wavelength. This is why the colors separate: each one is refracted at a slightly different angle. When the colors emerge out of the prism, they have fanned out into a beautiful spectrum, displaying all the colors from violet to red.

This separation of colors is what we call the dispersion of white light. It’s a simple yet profound demonstration that white light is made up of a spectrum of colors, and it’s the glass prism that acts as the tool to reveal this hidden secret.

Factors Influencing Dispersion of Light:

Dispersion is the process where white light separates into its component colors. This happens when light passes through a medium like a prism. But what determines how much each color spreads out? That’s where the factors influencing dispersion come into play.

Refractive Index: The refractive index of a medium is a measure of how much it can bend light. Materials with a higher refractive index will bend light more, causing greater dispersion. Think of it like running on a track; if the track suddenly becomes softer, you’ll slow down more and your path will bend.

The wavelength of Light: The wavelength of light is another crucial factor. Colors with shorter wavelengths (like blue and violet) are dispersed more than colors with longer wavelengths (like red). It’s similar to how a small car can make sharper turns compared to a long bus.

The angle of Incidence: The angle of incidence is the angle at which light hits the prism. This angle affects how much the light is bent inside the prism. If the light enters at a steeper angle, the dispersion will be more pronounced, just as a ball hitting a wall at a sharp angle bounces off further away.

Material Composition: The material composition of the prism also plays a role. Different materials have different capacities to disperse light. For example, a diamond prism will disperse light more than a glass prism because of its unique internal structure.

Prism Shape: Lastly, the shape of the prism itself can influence dispersion. A prism with a more acute angle will spread the colors out more than a prism with a wider angle. This is because the light spends more time inside the prism, which increases the effect of dispersion.

By understanding these factors, students can get a clearer picture of why dispersion occurs and how it behaves under different conditions.

Refraction of Light through Prism

Refraction is the bending of light as it passes from one transparent medium into another. This happens because light travels at different speeds in different mediums.

When a beam of white light hits a glass prism, it enters from the air (a rarer medium) into a glass (a denser medium). As it enters the glass, the light slows down and bends towards the normal line—a line perpendicular to the surface at the point of contact.

Once inside the prism, the light continues to travel, bending further as it passes through the glass. The amount of bending depends on the angle of incidence (the angle at which the light hits the prism) and the material’s refractive index (a number that describes how much the material can bend light).

As the light exits the prism, it moves from the glass back into the air. It speeds up again and bends away from the normal line. This change in speed and direction is what causes the light to refract. The path that the light takes through the prism is not straight. Instead, it’s a zigzag path due to the bending at both the entry and exit points. The overall effect is that the light has changed direction from its original path—this change is the angle of deviation .

Light bends because it follows the law of refraction , also known as Snell’s Law . This law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant and is equal to the refractive index. Refraction of light through a prism is a process where light bends as it passes from one medium to another, changing speed due to the different optical densities. This bending is responsible for the path of light being altered as it travels through the prism.

Visible Light Spectrum

The visible light spectrum is the range of electromagnetic waves that we can see. Each color has a different wavelength, and together they make up the light that brightens our world and allows us to enjoy the beauty of a rainbow or a sunset. The visible light spectrum is the range of light wavelengths that the human eye can see, approximately from 380 nm (violet) to 750 nm (red).

The visible light spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. It consists of a range of wavelengths that we perceive as different colors. The spectrum includes the colors red, orange, yellow, green, blue, indigo, and violet.

Each color in the visible spectrum has a specific wavelength range:

Our eyes have receptors called cones that are sensitive to these wavelengths. When light enters our eyes, the cones translate these wavelengths into electrical signals that our brain interprets as colors. The visible spectrum is continuous, meaning there are no gaps between the colors. They blend seamlessly into one another, creating the full array of colors we can see. Just outside the visible spectrum are ultraviolet light, which has shorter wavelengths, and infrared light, which has longer wavelengths. These are not visible to the naked eye but can be detected with special instruments.

Angle of Deviation

The angle of deviation is the angle between the direction of the incoming light and the direction of the light as it exits the prism. Each color has a different angle of deviation due to its unique wavelength. The angle of deviation is a measure used in optics to describe how much a beam of light is bent, or deviated, as it passes through a prism.

Imagine a football player running straight towards a goal. If the player encounters an obstacle that makes them change direction, the angle between their original path and their new path is similar to the angle of deviation in light. The greater the obstacle’s influence, the larger the deviation.

In technical terms, the angle of deviation (usually denoted by the Greek letter \(\displaystyle\delta \)) is the angle made between the incident ray of light entering the first face of the prism and the refracted ray that emerges from the second face of the prism. It’s like measuring the change in direction from where the light first hits the prism to where it comes out on the other side.

The angle of deviation is important because it tells us how much each color of light is bent by the prism. Since different colors of light have different wavelengths, they are deviated by different amounts. Violet light, for example, is bent more than red light.

The reason for this difference in bending is due to the light’s wavelength. Shorter wavelengths (like violet) are bent more, and longer wavelengths (like red) are bent less. This is why we see a spectrum of colors when white light passes through a prism – each color is deviated at a slightly different angle.

The exact angle of deviation depends on the prism’s material and the wavelength of light. It can be calculated using Snell’s Law and the prism’s geometry. For a simple glass prism, the formula to find the angle of deviation for a particular wavelength involves the refractive index of the material and the angle at which the light enters the prism.

The Angle of Deviation for White Light through a Prism:

The angle of deviation is the measure of how much a ray of light has been bent from its original path after passing through a prism. For white light, which is a mix of all visible colors, each color deviates by a different amount because each color has a different wavelength.

When white light enters a prism, it’s like a group of athletes, each running at a different speed, entering a field with a barrier. The athletes represent different colors of light, and the barrier represents the prism. As they cross the barrier, each one changes direction slightly. The amount they change direction, or their angle of deviation, depends on their speed, just like it depends on the wavelength of light.

Violet light, with the shortest wavelength, is bent the most, while red light, with the longest wavelength, is bent the least. This spread of colors is what creates a spectrum. The angle of deviation is largest for violet and smallest for red, with all the other colors falling in between.

The shape of the prism also affects the angle of deviation. A triangular prism with non-parallel sides will cause an overall angle of deviation for the various colors of white light, spreading them out to form a spectrum. The exact angle of deviation for each color can be calculated using Snell’s Law and the geometry of the prism.

Interestingly, there is a particular angle at which the light enters the prism where the deviation is the smallest for all colors. This is known as the angle of minimum deviation, and it occurs when the light travels symmetrically through the prism, with the path inside the prism being parallel to the base. This angle is unique for each color due to their different wavelengths.

The angle of deviation for white light through a prism is a beautiful demonstration of how light’s inherent properties, combined with the geometry of the prism, can lead to the colorful display we know as the spectrum.

Newton’s Prism Experiment

Sir Isaac Newton’s prism experiment was a pivotal moment in the scientific study of light and color. In 1666, Newton conducted a series of experiments to delve into the nature of light. Newton’s experiment showed that a prism could decompose white light into a spectrum of colors and that these colors could be recombined to form white light again.

Newton darkened his room and made a small hole in his window shutter, allowing just a single beam of sunlight to enter. He then placed a triangular glass prism in the path of the sunlight. As the sunlight passed through the prism, it spread out into a band of colors on the opposite wall. This band, known as a spectrum, displayed all the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet.

Newton observed that the prism did not create the colors; rather, it separated the colors that were already present in the sunlight. He realized that white light is a mixture of all the colors of the visible spectrum. To confirm his hypothesis, Newton performed another experiment. He placed a second prism upside down in front of the spectrum created by the first prism. Instead of creating a new set of colors, the second prism recombined the spectrum back into white light.

Newton concluded that white light is composed of different colors, and these colors can be separated and recombined by refraction through prisms. His experiment fundamentally changed our understanding of light and laid the foundation for the field of optics. Newton’s experiment with prisms showed that light is a complex entity made up of various colors, which can be bent and separated by refraction. This experiment is a classic example of how simple observations can lead to profound scientific discoveries.

Also Read: Total Internal Reflection

Some Natural Phenomena Due to Sunlight

A rainbow is a perfect example of light dispersion that we can observe in the sky. It occurs when sunlight interacts with raindrops after a storm. When sunlight enters a raindrop, it slows down and bends—a process known as refraction. Because sunlight is made up of different colors, each color bends at a slightly different angle due to its unique wavelength. Violet light bends the most, and red light bends the least.

A rainbow is a spectacular example of sunlight being dispersed by water droplets in the atmosphere. This phenomenon occurs due to the combined effects of dispersion, refraction, and reflection of sunlight by the spherical water droplets of rain.

Conditions for Observing a Rainbow: The Sun must be shining in one part of the sky, for example, near the western horizon. It should be raining in the opposite part of the sky, such as the eastern horizon. An observer can only see a rainbow with their back facing the Sun.

Figure (a) helps us understand the steps involved in the formation of a rainbow: When sunlight enters a raindrop, it is refracted (bent). This refraction causes the different wavelengths (colors) of white light to spread out. Longer wavelengths like red are bent the least, while shorter wavelengths like violet are bent the most.

These separated colors then strike the inner surface of the water droplet. If the angle of incidence is greater than the critical angle (48° for water), the light undergoes internal reflection.

The reflected light exits the droplet, undergoing another refraction. This further separates the colors. For example, violet light exits the raindrop at an angle of 40° relative to the incoming sunlight, and red light exits at an angle of 42°. Other colors emerge at angles between these two values.

Figure (b) shows how a primary rainbow forms: Red light from one droplet (drop 1) and violet light from another droplet (drop 2) reach the observer’s eye. Violet light from drop 1 and red light from drop 2 are directed above or below the observer.

The observer sees a rainbow with red on the top and violet on the bottom because of the angles at which these colors emerge from the droplets. The primary rainbow results from a three-step process: refraction, reflection, and refraction.

Figure (c) explains the formation of a secondary rainbow: A secondary rainbow occurs when light undergoes two internal reflections inside a raindrop instead of one. The secondary rainbow is fainter because some light intensity is lost during the second reflection. The order of colors is reversed compared to the primary rainbow, with red on the inner edge and violet on the outer edge.

The arc shape of a rainbow is due to the angle at which the sunlight is refracted and then reflected inside the raindrops. The light that reaches your eyes from the higher raindrops appears red, while the light from the lower raindrops appears violet, with all the other colors in between.

The colors of a rainbow always appear in the same order: red, orange, yellow, green, blue, indigo, and violet. This sequence is determined by the wavelengths of the colors, with red having the longest wavelength and violet the shortest.

Scattering Of Light

Scattering of light occurs when light rays encounter small particles in the atmosphere, such as air molecules, dust, or water droplets. These particles cause the light to change direction and spread out. During sunset and sunrise, the sun is near the horizon, and sunlight travels through a longer path in the atmosphere. This longer journey means the light encounters more particles, leading to more scattering.

The phenomenon responsible for the scattering of light in the atmosphere is known as Rayleigh scattering . It’s more effective at shorter wavelengths, such as blue and violet light, which is why the sky appears blue during the day. However, at sunrise and sunset, the situation is a bit different.

As the sun’s position is low, the sunlight’s path through the atmosphere is the longest. The blue and violet light is scattered out of the direct line of sight, and the longer wavelengths like red, orange, and yellow are less affected by scattering. This is why we often see a reddish sky at these times—the light that reaches us directly has more of the longer wavelengths.

Rayleigh’s law quantitatively describes the scattering of light, stating that the intensity of scattered light is inversely proportional to the fourth power of the wavelength. This means shorter wavelengths scatter much more than longer ones.

When light encounters particles in the atmosphere, it can be scattered. The nature of this scattering depends on the size of the particles relative to the wavelength of the light. The key factor here is the size of the particle (denoted as (a) compared to the wavelength of light (denoted as (λ). When the particles are much smaller than the wavelength of light (a << λ), we observe Rayleigh scattering .

Rayleigh scattering is more effective at shorter wavelengths. This is why the sky appears blue during the day; shorter (blue) wavelengths are scattered more than longer (red) wavelengths. The intensity of the scattered light (I) is inversely proportional to the fourth power of the wavelength (λ), which can be expressed as:

\(\displaystyle I \propto \frac{1}{\lambda^4} \)

For larger particles, such as dust and water droplets, the scattering is different. These particles are closer in size to the wavelength of visible light or even larger. In such cases, the scattering is less wavelength-dependent and can scatter light of all colors more or less equally. This is why clouds, which consist of water droplets, appear white; they scatter all wavelengths of light similarly.

Example: At sunset, sunlight travels through more of the Earth’s atmosphere, increasing the distance light travels through air and the number of particles it encounters. The increased scattering of shorter wavelengths of light (blue and violet) out of the line of sight leaves the longer wavelengths (red, orange, yellow) to reach the observer’s eyes, leading to the beautiful reds and oranges of a sunset.

The scattering of light in the atmosphere can vary depending on the size of the atmospheric particles relative to the wavelength of light. Rayleigh scattering dominates when the particles are much smaller than the wavelength, leading to the blue sky we see during the day, while larger particles scatter light more uniformly, contributing to the white appearance of clouds and the colors of sunsets.

Examples of Dispersion of Light

Rainbows : A rainbow is one of the most beautiful natural examples of light dispersion. After it rains, sunlight shines into droplets of water left in the air. Each droplet acts like a tiny prism, dispersing the sunlight into a spectrum of colors that arc across the sky.

Prisms : When white light passes through a glass prism , it is dispersed into a spectrum of colors. This happens because the different wavelengths of light are refracted by different amounts as they pass through the prism, separating the light into its constituent colors.

Soap Bubbles : The colorful patterns on soap bubbles are caused by dispersion. Light reflects off the different layers of soap film, and the varying thickness of the film causes different colors to be seen due to the dispersion of light.

CDs and DVDs : The surface of a CD or DVD can act like a prism. When light hits the surface, it’s dispersed into a rainbow of colors. This is due to the microscopic grooves on the disc diffracting the light and creating a spectrum.

Oil on Water : A thin layer of oil on water can create a rainbow effect. The light is dispersed by the varying thickness of the oil, which acts like a prism, separating the light into different colors.

These examples show how the dispersion of light is not just a concept in textbooks but a phenomenon that surrounds us, adding color to the natural world. It’s a principle that explains why we see such a variety of colors in different situations and helps students understand the practical implications of the theories they learn in class.

Practical Applications of Dispersion of Light

The dispersion of light, the process in which white light separates into its component colors, has several practical applications that are fascinating and integral to our daily lives. Here are some key applications explained simply:

• Spectroscopy: Spectroscopy is a technique that uses the dispersion of light to analyze the composition of materials. Scientists can identify the substance’s chemical makeup by passing light through a substance and examining the spectrum of colors produced.
• Optical Instruments: Prisms are used in various optical instruments like spectrometers and telescopes to disperse light into its constituent colors. This dispersion is crucial for understanding the properties of light from different celestial bodies.
• Vision Correction: Prism spectacles utilize dispersion to correct certain vision problems. These glasses have prisms that adjust the light entering the eyes, helping to correct alignment issues and improve visual clarity.
• Laser Tuning: In laser technology, dispersion is used to tune lasers to emit specific colors or wavelengths. This is essential in applications ranging from medical treatments to data transmission.
• Rainbow Formation: The natural phenomenon of rainbows is an example of dispersion. Sunlight disperses through water droplets in the atmosphere, creating the colorful arc we see in the sky after rain.
• Art and Decoration: The colorful patterns seen on CDs, soap bubbles, or oil spills on water are due to the dispersion of light. These effects are often used for artistic purposes or in decorative items.
• Environmental Monitoring: Dispersion can also be used to monitor environmental changes. For instance, changes in the dispersion patterns of sunlight can indicate the presence of pollutants in the atmosphere.

These applications show how the fundamental concept of light dispersion is applied in various fields, enhancing our scientific understanding and contributing to practical solutions in everyday life.

Also Read: Refraction Through A Prism

Q: What happens if you use a material other than glass for the prism?

When a material other than glass is used for a prism, the dispersion of light—that is, the separation of white light into its constituent colors—can vary significantly. This variation is primarily due to differences in the material’s optical density and refractive index, which are key factors in how much light bends when passing through the prism.

The optical density of a material is a measure of how much it slows down light. A material with a higher optical density will slow down light more, leading to a greater bending or refraction of the light rays. This means that materials with higher optical densities will generally cause more dispersion.

The refractive index is a number that describes how much a material can bend light. It varies with the wavelength or color of the light, a phenomenon known as dispersion . Materials with higher refractive indices will disperse light more than those with lower refractive indices. For example, a diamond has a higher refractive index than glass and thus will disperse light into a more vivid spectrum.

Different materials will produce different dispersion characteristics. For instance:

• Diamond : Known for its high dispersion, resulting in a very colorful spectrum.
• Flint Glass : Has a higher dispersion than standard crown glass, leading to a wider spectrum.
• Acrylic : Typically has a lower refractive index than glass so it would produce less dispersion.

The spectrum quality produced by a prism also depends on the material. Some materials may produce a sharper, more defined spectrum, while others might result in a more blurred or less distinct spectrum.

The choice of material for a prism is often based on the desired application. For broad-spectrum spectroscopy, where a wide range of wavelengths needs to be covered, materials that provide good dispersion without too much absorption are preferred.

Using different materials for prisms affects the degree and quality of light dispersion. The choice of material will depend on the specific needs of the application, such as the required precision of the spectral lines and the range of wavelengths to be analyzed.

Q: Can you explain the concept of critical angle in non-glass prisms?

The critical angle is the angle of incidence above which light is internally reflected within a material. When light travels from a denser medium to a rarer medium (like from water to air), there’s a specific angle of incidence at which the refracted ray of light skims the surface. This specific angle is known as the critical angle.

For non-glass prisms, such as those made of water, acrylic, or diamond, the critical angle will differ because each material has a unique refractive index. The refractive indices of the two media at the interface determine the critical angle.

The critical angle (θ c ) can be calculated using Snell’s Law, which relates the angle of incidence (θ i ) to the angle of refraction (θ r ). The formula for the critical angle is:

\(\displaystyle \theta_c = \sin^{-1}\left(\frac{n_r}{n_i}\right) \)

where (n i ) is the refractive index of the denser medium (inside the prism) and (n r ) is the refractive index of the rarer medium (outside the prism).

For example, if you have a prism made of water (with a refractive index of about 1.33) in air (with a refractive index of about 1.00), the critical angle can be calculated as:

\(\displaystyle \theta_c = \sin^{-1}\left(\frac{1.00}{1.33}\right) \)

which gives a critical angle of about 48.6 degrees. Any light hitting the water-air interface at an angle greater than 48.6 degrees will be internally reflected.

The critical angle is a fundamental concept in optics that depends on the refractive indices of the materials involved. It’s the angle beyond which light cannot pass through the interface but is instead reflected into the material, and it varies for different materials used in prisms.

Q: How does dispersion change with irregular-shaped prisms?

Dispersion of light in irregular-shaped prisms can be quite different from what we observe in regular triangular prisms. In a regular prism, the path of light is predictable, with light entering and exiting at specific angles. In an irregular-shaped prism, the angles can vary greatly, leading to a more complex path for the light as it travels through the prism.

The angle of refraction, which is the angle at which light bends when entering or exiting the prism, can differ significantly in irregular shapes. This means that the angle of deviation for each color of light could be less uniform compared to a triangular prism.

The spread of the spectrum can also be affected. In a triangular prism, the spectrum is usually linear and orderly. In an irregular-shaped prism, the spectrum could be spread out in a non-linear fashion, potentially creating a more scattered pattern of colors. Due to the varying angles and surfaces, some colors may overlap or mix, leading to a less distinct separation of the spectrum. This can result in a blending of colors rather than a clear division between them.

The intensity of the dispersed colors might also change. Some colors may appear brighter or more pronounced, while others could be less visible, depending on the shape of the prism and how it affects the light’s path. Irregular shapes can cause light to pass through varying thicknesses of the prism material, which can alter the refractive index experienced by different light rays. This can further influence the dispersion pattern.

A regular prism produces a predictable and orderly spectrum, an irregular-shaped prism can create a more complex and less predictable pattern of dispersed light. The exact nature of the dispersion will depend on the specific shape and material of the prism.

Q: How does the angle of deviation change with different prism shapes?

The angle of deviation changes with different prism shapes due to the varying angles at which light enters and exits the prism, and the path it takes through the material. Here’s how different shapes influence the angle of deviation:

• Triangular Prisms : For a triangular prism, which is the most common type used to demonstrate dispersion, the angle of deviation depends on the refractive index of the material and the prism’s apex angle. The apex angle is the angle between the two faces of the prism through which light enters and exits. A larger apex angle generally results in a larger angle of deviation because the light spends more time inside the prism, increasing the opportunity for refraction.
• Rectangular Prisms : Rectangular prisms can also cause deviation of light, but since the angles at which light enters and exits are usually 90 degrees, there is no dispersion, and the deviation is minimal. The light path is essentially parallel-shifted.
• Pentagonal Prisms : Pentagonal prisms have more faces, which means that light can undergo multiple refractions. This can lead to a complex path of light within the prism, potentially increasing the angle of deviation depending on the arrangement of the faces.
• Prism Material : The material of the prism also affects the angle of deviation. Different materials have different refractive indices, which means that the same shape of prism made from different materials will deviate light by different amounts.
• The angle of Incidence : The angle at which light enters the prism, known as the angle of incidence, also plays a significant role. If the light enters at a steeper angle relative to the prism’s surface, the angle of deviation will be larger.

So, the angle of deviation is influenced by the shape of the prism, the material’s refractive index, and the angle of incidence. Each of these factors can alter the path of light through the prism, resulting in different angles of deviation for different prism shapes

What is the dispersion of white light by a glass prism and why does it occur?

Dispersion of white light by a glass prism occurs when white light passes through the prism and splits into its constituent colors. This happens because different colors (wavelengths) of light refract by different amounts as they pass through the prism. The variation in the refractive index for different wavelengths causes this separation, with violet light bending the most and red light bending the least.

What is the visible light spectrum, and what are its primary colors?

The visible light spectrum is the range of electromagnetic waves that are visible to the human eye, typically ranging from approximately 400 nm (violet) to 700 nm (red). The primary colors in the visible spectrum, in order of increasing wavelength, are violet, indigo, blue, green, yellow, orange, and red.

What was Newton’s prism experiment and what did it demonstrate about light?

Newton’s prism experiment involved passing a beam of sunlight through a glass prism, which resulted in the dispersion of light into a spectrum of colors. He further demonstrated that by passing the dispersed light through a second prism, the colors could be recombined to form white light again. This experiment proved that white light is composed of multiple colors and that prisms can separate and recombine these colors through refraction.

How are rainbows formed, and what role does dispersion play in their formation?

Rainbows are formed when sunlight passes through raindrops in the atmosphere. Each raindrop acts like a prism, refracting and internally reflecting the light, which then exits the drop and disperses into its constituent colors. The dispersion of light within the raindrop separates the colors, creating the circular spectrum observed in a rainbow.

What causes the scattering of light and why is the sky blue?

The scattering of light is caused by the interaction of light with small particles in the atmosphere. Shorter wavelengths of light (blue and violet) are scattered more than longer wavelengths (red and orange). Although violet light is scattered more, our eyes are more sensitive to blue light, and some violet light is absorbed by the upper atmosphere, making the sky appear blue during the day.

What is the phenomenon of a double rainbow and how does it occur?

A double rainbow occurs when light is reflected twice inside raindrops before emerging. The second reflection causes the formation of a secondary rainbow with colors reversed and appearing outside the primary rainbow. This secondary bow is usually fainter due to the extra reflection reducing the intensity of the light.

How does the scattering of light explain the reddish color of the sunset?

The reddish color of the sunset is explained by Rayleigh scattering. During sunset, the sun’s light has to pass through a greater thickness of the Earth’s atmosphere, which scatters shorter wavelengths (blue and violet) out of the direct path. This leaves the longer wavelengths (red and orange) to dominate the sky’s color, creating a reddish appearance at sunset.

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The science of color, newton’s rainbow.

In the 1660s, English physicist and mathematician Isaac Newton began a series of experiments with sunlight and prisms. He demonstrated that clear white light was composed of seven visible colors.

By scientifically establishing our visible spectrum (the colors we see in a rainbow), Newton laid the path for others to experiment with color in a scientific manner. His work led to breakthroughs in optics, physics, chemistry, perception, and the study of color in nature.

Aristotle developed the first known theory of color, suggesting that all colors came from white and black (lightness and darkness) and related them to the four elements – water, air, earth, and fire. Aristotle’s beliefs on color were widely held for over 2000 years until being replaced by those of Newton.

Opticks , one of the great works in the history of science, documents Newton’s discoveries from his experiments passing light through a prism. He identified the ROYGBIV colors (red, orange, yellow, green, blue, indigo, and violet) that make up the visible spectrum. The visible spectrum is the narrow portion within the electromagnetic spectrum that can be seen by the human eye. Other forms of electromagnetic radiation, waves of energy, that we cannot see include radio, gamma and microwaves. The cells in our eyes called cones are sensitive to the wavelengths found in the visible spectrum. They allow us to see the all the colors of the rainbow.

…if the Sun’s Light consisted of but one sort of Rays, there would be but one Colour in the whole World… –Sir Isaac Newton, Opticks

Goethe challenged Newton’s views on color, arguing that color was not simply a scientific measurement, but a subjective experience perceived differently by each viewer. His contribution was the first systematic study on the physiological effects of color. Goethe’s views were widely adopted by artists. Although Goethe is best known for his poetry and prose, he considered Theory of Colors his most important work.

Colour are light’s suffering and joy. –Johann Wolfgang von Goethe

This very rare book formed the foundation for modern color printing. Le Blon was the first to outline a three-color printing method using primary colors (red, yellow, blue) to create secondary colors (green, purple, orange). He makes an important distinction between “material colors,” as used by painters, and colored light, which was the focus of Newton’s color theories. Le Blon’s distinction marks the first documentation of what is now referred to as additive and subtractive color systems. Rainbows, TVs, computer screens and mobile devices all emit light and are examples of an additive color system (the subject of Newton’s Opticks). Red, green and blue are the primary additive colors and when combined they produce transparent white light. Books, paintings, grass and cars are examples of a subtractive color system which is based on the chemical makeup of an object and its reflection of light as a color. Subtractive primary colors - blue, red, and yellow – are often taught to us as children, and when mixed together they create black.

…I arriv’d at the skill of reducing the Harmony of Colouring in painting to Mechanical Practice… –J.C. Le Blon, Coloritto

These colorful line diagrams reveal the chemical compositions of metals. When a pure metal is burned and viewed through a spectroscope, each element gives off unique spectra, a sort of color fingerprint. This method, called spectral analysis, led to the discovery of new elements, and marked the first steps towards quantum theory.

Can you see the numbers in the circles? 4.5 percent of the population cannot see the entire visible spectrum, a condition called color vision deficiency, or color blindness. Ishihara plates are used to test patients for the various types of color blindness.

Can you find the animal hiding in this image? Camouflage uses color to conceal forms by creating optical illusions. American artist Abbott Thayer introduced the concept of  disruptive patterning , in which an animal’s uneven markings can disguise its outline. In this illustration Thayer shows how a peacock can disappear into its surroundings.

Thayer, an American artist, devoted much of his life to understanding how animals conceal themselves in nature for survival. In his book, Concealing Coloration in the Animal Kingdom, Thayer presented his beliefs of protective coloration as an essential factor in evolution helping animals disguise themselves from predators. He received much praise and criticism. He was extreme in his views arguing that all animal coloration was for protective purposes and failing to recognize other possible reasons such as sexual selection – characteristics for attracting a mate.  Teddy Roosevelt most notably attacked his theories by pointing out that this concealment doesn’t last all season, or even all day, but was dependent on a single frozen moment in times. Despite these shortcomings, Thayer went on to be the first to propose camouflage for military purposes.  Although his suggestions were initially rejected, his former students were among the founders of the American Camouflage Society in 1916 and his theories were eventually adopted and are still used today.

The colorful pattern on this German aircraft from World War I is called lozenge camouflage. Its disruptive pattern applied Abbott Thayer’s theories in an effort to inhibit enemy observation from the air and on the ground.

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• 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.

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.

White light is incident on a 60°, 60°, 60° prism. Use the slider to change the initial angle of incidence.

You can set the refractive index for red light (n r ) and the difference in indices for red and blue light (n b -n r ).

The angle of dispersion is the angle between the red and blue rays after they have refracted through the prism.

The angle of deviation is the angle between the line of the incident ray and the final refracted red ray.

Angular Dispersion

Deduction of angular dispersion in thin prism :

The thin prism is always in the position of minimum deviation , It disperses the white light into the seven spectral colours , where :

The angle of deviation of the red light is estimated from the relation :

 ( α o ) r = A ( n r – 1 )

The angle of deviation of the blue light is estimated from the relation :

 ( α o ) b = A ( n b – 1 )

Where : ( n r ) is the prism’s refractive index for red light and ( n b ) is the prism’s refractive index for blue light .

 ( α o ) b − ( α o ) r = A ( n b – n r )

The value ( α o ) b − ( α o ) r is called the angular dispersion between the blue and the red rays.

Factors affect the angular dispersion :

The apex angle of the prism .

The prism’s refractive index for both blue and red colours .

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. 

(Image will be uploaded soon)

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 this 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. 

The dispersion of white light happens because of the angle of refraction. The process of refraction is defined as the blending of light when it passes from one medium to another medium. The light deviates twice on passing through the glass prism, initially when it enters the prism and the second time when it comes out of it. Since all the colours have different wavelengths and are refracted from different frequencies and deviation angles, hence the Violet colour blends the most and red the least. 

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.

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.

Type of Dispersion Prisms

Equilateral Dispersion Prism: 60°- 60°- 60°

Isoscele Prism: 30°- 60°- 90°

Littrow Prism: 30°- 60°- 90° with one side HR coating

Pellin-Broca Prism: 90° -75° 135° -60°, features with Brewster angle input and output, and bends light by 90°. It is good for laser beam separation such as SHG, THG.

Ultrafast Laser Dispersion Prism Pair - These Dispersion Prism Pairs are used to compensate for spectral dispersion that occurs in ultrafast laser systems. The prism pairs are matched to within a few arcseconds and are designed so that the input and output angles are both at Brewster's Angle.

Compound Dispersion Prism such as Amici Prism - A compound prism is a set of multiple triangular prism elements placed in contact, and often cemented together to form a solid assembly. 

Rainbow is an Example of Dispersion of Sunlight

After a rain-shower, water droplets suspended in the atmosphere act like tiny prisms for the sunlight. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. We see the colours in a rainbow because of the dispersion of sunlight inside a raindrop. 

Two essential conditions for a rainbow to form:

The sun should at the opposite side of the viewing direction,

Suspended water droplets must be present in the air.

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.

FAQs on Prism Dispersion

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?

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.

3. What Causes Dispersion of Prism?

Dispersion of prism takes place because white light entering the prism consists of so many different colors. Each of these different colors has a different wavelength. According to Cauchy’s formula, refractive index (μ) of a material depends upon wavelength  (λ) and is given by,

\[\mu  = a+ b(\lambda ^{2})+ \frac{c}{(\lambda ^{4})} \], where a, b, c are constants of the material.

The wavelength of violet light is smaller than that of the red light μ v > μ r, therefore the violet light has a larger angle than the red light. As a result, the dispersion of white light takes place on the second surface of the prism.

4. How is the rainbow formed?

The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion of light and internal reflection, different colours reach the observer’s eye. Red colour appears on top and violet at the bottom of the rainbow. A rainbow is always formed in a direction opposite to that of the Sun. At ‘A’ – Refraction and dispersion take place. At ‘B’ – Internal reflection takes place. At ‘C’ – Refraction and dispersion take place.

5. Why should students study with study materials on Prism Dispersion from Vedantu?

Vedantu is one of the study platforms for students to accelerate their exam preparation as it provides an ample range of study materials for physics subjects including worksheets, sample papers, important theories, previous years’ question papers, important questions as well as RS Aggarwal solutions, RD Sharma Solutions, and NCERT solutions for all classes. Students can also browse through online tutorials and interactive lessons provided on Vedantu to boost their exam preparation and get higher marks in physics board exams as well as school exams.