Refraction & Dispersion in a Raindrop
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This is one of a set of almost 40 diagrams exploring Rainbows.
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Description
Refraction & Dispersion in a Raindrop
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About the diagram
Overview of raindrops
An idealized raindrop forms a sphere. These are the ones that are favoured when drawing diagrams of both raindrops and rainbows because they suggest that when light, air and water droplets interact they produce predictable and replicable outcomes.
- In real-life, full-size raindrops don’t form perfect spheres because they are composed of water which is fluid and held together solely by surface tension.
- In normal atmospheric conditions, the air a raindrop moves through is itself in constant motion, and, even at a cubic metre scale or smaller, is composed of areas at slightly different temperatures and pressure.
- As a result of turbulence, a raindrop is rarely in free-fall because it is buffeted by the air around it, accelerating or slowing as conditions change from moment to moment.
- The more spherical raindrops are, the better defined is the rainbow they produce because each droplet affects incoming sunlight in a consistent way. The result is stronger colours and more defined arcs.
Real-life raindrops
- Raindrops start to form high in the atmosphere around tiny particles called condensation nuclei — these can be composed of particles of dust and smoke or fragments of airborne salt left over when seawater evaporates.
- Raindrops form around condensation nuclei as water vapour cools producing clouds of microscopic droplets each of which is held together by surface tension and starts off roughly spherical.
- Surface tension is the tendency of liquids to shrink to the minimum surface area possible as their molecules cohere to one another.
- At water-air interfaces, the surface tension that holds water molecules together results from the fact that they are attracted to one another rather than to the nitrogen, oxygen, argon or carbon dioxide molecules also present in the atmosphere.
- As clouds of water droplets begin to form, they are between 0.0001 and 0.005 centimetres in diameter.
- As soon as droplets form they start to collide with one another. As larger droplets bump into other smaller droplets they increase in size — this is called coalescence.
- Once droplets are big and heavy enough they begin to fall and continue to grow. Droplets can be thought to be raindrops once they reach 0.5mm in diameter.
- Sometimes, gusts of wind (updraughts) force raindrops back into the clouds and coalescence starts over.
- As full-size raindrops fall they lose some of their roundness, the bottom flattens out because of wind resistance whilst the top remains rounded.
- Large raindrops are the least stable, so once a raindrop is over 4 millimetres it may break apart to form smaller more regularly shaped drops.
- In general terms, raindrops are different sizes for two primary reasons, initial differences in particle (condensation nuclei) size and different rates of coalescence.
- As raindrops near the ground, the biggest are the ones that bump into and coalesce with the most neighbours.
About the diagram
This diagram considers what happens to a ray of incident light that contains wavelengths corresponding with red, green and blue when it strikes a raindrop.
- It shows that refraction causes chromatic dispersion as each wavelength changes direction by a different amount.
- The effects of refraction, reflection and dispersion all help to explain why rainbows appear when sunlight strikes falling rain.
- Remember that when light strikes the boundary between two different media it may be partially reflected and partially refracted.
- If both reflection and refraction take place:
- A proportion of the light bounces off the surface of the new medium and returns into the medium from which it originated.
- A proportion crosses the boundary and undergoes refraction, so changes speed and direction.
- To delve a bit further into the diagram let’s go on to review the three key concepts, refraction, chromatic dispersion and scattering.
Refraction
Chromatic dispersion
Chromatic dispersion means dispersion according to colour and associated wavelengths of light. Under certain conditions, chromatic dispersion causes light to separate into its component wavelengths producing a rainbow of colours for a human observer.
- Chromatic dispersion is best demonstrated by passing a beam of light through a glass prism.
- A familiar example of chromatic dispersion is when white light strikes raindrops and a rainbow of colours becomes visible to an observer.
- As light first enters and then exits each raindrop, it separates into its component wavelengths which the observer sees as a band of distinct colours.
- Chromatic dispersion can be explained in terms of the relationship between wavelength and refractive index.
- When light propagates from one medium (such as air) to another (such as glass or water) every wavelength of light is affected to a different degree according to the refractive index of the media concerned. As a result, each wavelength changes direction by a different degree. In the case of white light, the separate wavelengths fan out with red on one side and violet on the other.
- Remember that wavelength is a property of electromagnetic radiation, whilst colour is a feature of visual perception.
Scattering
Scattering takes place when streams of photons (or waves of light) are deflected in different directions. In this resource, the term is used to refer to the different forms of deviation produced by diffusion, dispersion, interference patterns, reflection and refraction as well as by the composition and surface properties of different media.
Regular scattering
- When light of a particular wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, the refractive indices of air and water and the surface properties of the droplet.
- For incident rays of a single wavelength striking the surface of a single droplet at different points, it is the different angles at which they enter the droplet that are the chief determinant of the way they scatter as they exit the droplet. In this case.
- For incident rays of a white light striking the surface of a single droplet at different points, it is the combined effects of the different angles at which they enter the droplet along with the effects of chromatic dispersion (causing the separation of white light into spectral colours) that determine the form of scattering.
- Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.
- Regular scattering is not random and obeys the law of reflection and refraction (Snell’s law).
Random scattering
- In optics, diffusion results from any material that scatters light during transmission or reflection producing softened effects without sharp detail.
- Objects produce diffuse reflections when light bounces off a rough or uneven surface and scatters in all directions.
- Transparent and translucent materials transmit diffuse light unless their surfaces are perfectly flat and their interiors are free of foreign material.
- All objects obey the law of reflection on a microscopic level, but if the irregularities on the surface of an object are larger than the wavelength of light, the light undergoes diffusion.
- A reflection that is free of the effects of diffusion is called a specular reflection.
- In the case of raindrops, random scattering can result from:
Definition
Explanation
Summary
About sections (temp)
References
Some key terms
A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.
- Rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
- Waterfalls, lawn sprinklers and other things that produce water droplets can produce a rainbow.
- A rainbow is formed from millions of individual droplets each of which reflects and refracts a tiny coloured image of the sun towards the observer.
- It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
- When the sun is behind an observer then the rainbow will appear in front of them.
Any material through which an electromagnetic wave propagates (travels) is called a medium (plural media).
- In optics, a medium is a material through which electromagnetic waves propagate.
- Although electromagnetic radiation is able to propagate through a wide range of media, it is not dependent upon on any medium for propagation and travels at the speed of light through a vacuum.
- The reason an electromagnetic wave does not need a medium to propagate through is because the only thing that is waving/oscillating is the value of the electric and magnetic fields.
- In general terms, empty space (a vacuum) is not considered to be a medium because it does not contain matter.
- It is the permittivity and permeability of a medium that determines how waves travel.
Summary
About sections (temp)
Dispersion (or chromatic dispersion) refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.
- Dispersion is the result of the relationship between refractive index and wavelength.
- Every wavelength of light is affected to a different degree by the refractive index of a medium. The result is that each wavelength changes direction and speed by a different amount.
- In the case of white light, the separate wavelengths span out with red at one end and violet at the other.
- A familiar example of dispersion is when white light strikes raindrops and a rainbow of colours becomes visible to an observer.
Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.
- The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
- The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
- Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
- Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
- Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.
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