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:
(image will be uploaded soon)
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:
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).
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.
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. What is refraction of light, and how does it differ from dispersion as per CBSE Class 12 Physics syllabus?
Refraction is the bending of light as it passes from one transparent medium to another due to a change in its speed. Dispersion, on the other hand, refers to the splitting of white light into its component colours when it passes through a prism. Refraction involves a change in the direction and speed of light, while dispersion shows the separation of colours based on their wavelengths.
2. Why do stars appear to twinkle, but planets do not, according to the concept of refraction?
Stars appear to twinkle because their light passes through layers of Earth’s atmosphere that have varying densities. The continuous change in refractive index causes the apparent position and brightness of starlight to fluctuate. Planets, being closer and apparent as extended sources, average out these fluctuations, so they do not twinkle.
3. What causes the formation of a rainbow, and what physical phenomena are involved?
A rainbow is formed when sunlight passes through water droplets suspended in the atmosphere after rain. These droplets act as tiny prisms that refract, reflect, and disperse the light. Due to refraction and dispersion, white light splits into seven constituent colours, creating the spectrum seen as a rainbow.
4. How does the refractive index of a medium affect the velocity and direction of light?
The refractive index (n) of a medium quantifies how much light slows down when passing through it compared to vacuum. A higher refractive index means light travels slower and bends more towards the normal. This change in velocity and direction is described mathematically by Snell's Law: n = c/v, where c is the speed of light in vacuum and v is the speed in the medium.
5. List real-life examples of dispersion of light observable in daily life or nature.
Common examples of dispersion of light include:
- Formation of rainbows after rainfall
- Colours seen in oil films on water
- Sparkling colours from a diamond
- Dewdrops displaying a spectrum
- Light pattern on a compact disc (CD)
6. Why does a glass prism split white light into different colours but a glass slab does not show the same effect prominently?
A glass prism splits white light because its non-parallel surfaces cause each colour to refract and deviate at different angles due to their varying wavelengths. In a glass slab, the parallel surfaces cause the separated colours to recombine as the light exits, so dispersion is not visible to the naked eye.
7. What are the practical applications of refraction in optical instruments?
Applications of refraction include:
- Formation of images in cameras and projectors
- Corrective lenses for vision (eyeglasses and contacts)
- Magnifying glasses
- Microscopes and telescopes
- Peepholes and binoculars
8. How does the wavelength of light relate to its deviation during dispersion through a prism?
Deviation of light in a prism is inversely proportional to its wavelength. Shorter wavelengths (violet) bend more than longer ones (red), which is why violet is at the bottom and red at the top of the spectrum formed by dispersion.
9. What is Snell's law, and why is it important for understanding refraction?
Snell’s law describes the relationship between the angle of incidence and angle of refraction when light passes from one medium to another. It is stated as: n₁ sin θ₁ = n₂ sin θ₂, where n represents the refractive indices and θ the respective angles. This law helps predict exactly how much light will bend when entering a new medium.
10. How do atmospheric temperature and pressure affect the refraction of light experienced in real-world phenomena?
Changes in temperature and pressure affect the density and refractive index of air, leading to varying degrees of light bending. At higher altitudes, lower pressure and temperature cause less refraction. These changes explain phenomena like the apparent bending of objects seen across a hot road or the shifting of star positions.
11. Compare reflection and refraction in terms of how light behaves at the boundary between two media.
Reflection occurs when light bounces back from the surface of a medium, with the angle of incidence equal to the angle of reflection. Refraction involves light passing into a different medium, changing direction due to a change in speed, resulting in a bent path where the angle of incidence typically does not equal the angle of refraction.
12. What are the types of dispersion seen in optical fibers, and how do they impact signal transmission?
In optical fibers, three main types of dispersion affect signal quality:
- Material (chromatic) dispersion: Different wavelengths travel at different speeds due to the fiber material.
- Modal (intermodal) dispersion: Light follows different paths or modes, causing parts of a signal to arrive at different times.
- Waveguide dispersion: Caused by the design and refractive index profile of the fiber core.
Dispersion causes signal pulse spreading, reducing the clarity of data transmission.
13. If the frequency of light does not change during refraction, what properties of light are altered when it enters a different medium?
When light undergoes refraction, its speed and wavelength change according to the refractive index of the new medium, but its frequency remains the same. This alteration leads to the bending of the light path without a change in its colour.
14. What conceptual mistake do students often make regarding the cause of dispersion in a prism?
A common misconception is that dispersion is caused solely by the shape of the prism. In reality, dispersion occurs because each colour in white light has a different wavelength and therefore a different refractive index, causing them to deviate at different angles, not just because the prism is triangular.
15. How is the concept of refraction utilised in correcting vision in humans?
Refraction is used in corrective lenses (concave or convex) to adjust the focus of light rays onto the retina. Depending on whether a person is nearsighted or farsighted, different lenses are used to properly bend light for clear vision.
