rainbow formation

8/3/2019 Rainbow Formation

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Rainbow Formation

One of nature's most splendid masterpieces is the rainbow. A rainbow is anexcellent demonstration of thedispersion of light and one more piece ofevidence thatvisible light is composed of a spectrum of wavelengths, eachassociated with a distinct color. To view a rainbow, your back must be to the

sun as you look at an approximately 40 degree angle above the ground into aregion of the atmosphere with suspended droplets of water or even a light

mist. Each individual droplet of water acts as a tiny prism that both dispersesthe light and reflects it back to your eye. As you sight into the sky, wavelengths of light associated

with a specific color arrive at your eye from the collection of droplets. The net effect of the vastarray of droplets is that a circular arc ofROYGBIVis seen across the sky. But just exactly how dothe droplets of water disperse and reflect the light? And why does the pattern always appear asROYGBIV from top to bottom? These are the questions that we will seek to understand on this page

of The Physics Classroom Tutorial. To understand these questions, we will need to draw upon ourunderstanding ofrefraction,internal reflection and dispersion.

The Path of Light Through a Droplet

A collection of suspended water droplets in the atmosphere serves as a refractor of light. The waterrepresents a medium with a different optical density than the surrounding air. Light waves refractwhen they cross over the boundary from one medium to another. The decrease in speed upon entryof light into a water droplet causes a bending of the path of light towards the normal. And upon

exiting the droplet, light speeds up and bends away from the normal. The droplet causes a deviationin the path of light as it enters and exits the drop.

There are countless paths by which light rays from the sun can pass through a drop. Each path ischaracterized by this bending towards and away from the normal. One path of great significance inthe discussion of rainbows is the path in which light refracts into the droplet, internally reflects, andthen refracts out of the droplet. The diagram at the right depicts such a path. A light ray from the

sun enters the droplet with a slight downward trajectory. Upon refracting twice and reflecting once,the light ray is dispersed and bent downward towards an observer on earth's surface. Other entrylocations into the droplet may result in similar paths or even in light continuing through the dropletand out the opposite side without significant internal reflection. But for the entry location shown in

the diagram at the right, there is an optimal concentration of light exiting the airborne droplet at anangle towards the ground. As in the case ofthe refraction of light through prisms with nonparallel

sides, the refraction of light at two boundaries of the droplet results in the dispersion of light into aspectrum of colors. The shorter wavelength blue and violet light refract a slightly greater amount

than the longer wavelength red light. Since the boundaries are not parallel to each other, the doublerefraction results in a distinct separation of the sunlight into its component colors.

The angle of deviation between the incoming light rays from the sun and the refracted rays directedto the observer's eyes is approximately 42 degrees for the red light. Because of the tendency of

shorter wavelength blue light to refract more than red light, its angle of deviation from the originalsun rays is approximately 40 degrees. As shown in the diagram, the red light refracts out of thedroplet at a steeper angle toward an observer on the ground. There are a multitude of paths bywhich the original ray can pass through a droplet and subsequently angle towards the ground.

Some of the paths are dependent upon which part of the droplet the incident rays contact. Other

paths are dependent upon the location of the sun in the sky and the subsequent trajectory of theincoming rays towards the droplet. Yet the greatest concentration of outgoing rays is found at these40-42 degree angles of deviation. At these angles, the dispersed light is bright enough to result in a

rainbow display in the sky. Now that we understand the path of light through an individual droplet,we can approach the topic of how the rainbow forms.

The Formation of the Rainbow
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A rainbow is most often viewed as a circular arc in the sky. An observer on the ground observes ahalf-circle of color with red being the color perceived on the outside or top of the bow. Those who

are fortunate enough to have seen a rainbow from an airplane in the sky may know that a rainbowcan actually be a complete circle. Observers on the ground only view the top half of the circle since

the bottom half of the circular arc is prevented by the presence of the ground (and the ratherobvious fact that suspended water droplets aren't present below ground). Yet observers in anairborne plane can often look both upward and downward to view the complete circular bow.

The circle (or half-circle) results because there are a collection of suspended droplets in theatmosphere that are capable concentrating the dispersed light at angles of deviation of 40-42degrees relative to the original path of light from the sun. These droplets actually form a circular

arc, with each droplet within the arc dispersing light and reflecting it back towards the observer.Every droplet within the arc is refracting and dispersing the entire visible light spectrum (ROYGBIV).As described above, the red light is refracted out of a droplet at steeper angles towards the groundthan the blue light. Thus, when an observer sights at a steeper angle with respect to the ground,

droplets of water within this line of sight are refracting the red light to the observer's eye. The bluelight from these same droplets is directed at a less steep angle and is directed along a trajectorythat passes over the observer's head. Thus, it is the red light that is seen when looking at thesteeper angles relative to the ground. Similarly, when sighting at less steep angles, droplets of

water within this line of sight are directing blue light to the observer's eye while the red light isdirected downwards at a more steep angle towards the observer's feet. This discussion explains why

it is the red light that is observed at the top and on the outer perimeter of a rainbow and the bluelight that is observed on the bottom and the inner perimeter of the rainbow.

Rainbows are not limited to the dispersion of light by raindrops. The splashing of water at the base

of a waterfall caused a mist of water in the air that often results in the formation of rainbows. Abackyard water sprinkler is another common source of a rainbow. Bright sunlight, suspendeddroplets of water and the proper angle of sighting are the three necessary components for viewingone of nature's most splendid masterpieces.

17- ISAAC NEWTON

The Nature of Colours

Isaac Newton was born at Woolsthorpe in Lincolnshire on Christmas Day, 1642. Hisfather had died before he was born, and his mother married again when he was only two.
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As a child he demonstrated his manual dexterity as he `busied himself making models of

wood in many kinds'. Most of his childhood was spent with his grandmother. He went

away to school at Grantham, and then on to Cambridge in 1661, but not before he hadtried his hand at farming without a great deal of enthusiasm.

Newton was very successful at Cambridge. He was elected to a minor Fellowship atTrinity College in 1667 and became a major Fellow in 1668. In 1669, at the age of

twenty-six, he was elected to the Lucasian Chair of mathematics.

The Great Plague had closed the university in 1665, and Newton retired to his mother's

farm at Woolsthorpe. His great productive period had begun in about 1664. The falling

apple that sparked off his theory of universal gravitation is said to have come from one of

the trees in the Woolsthorpe orchard. Between 1665 and 1667 he developed the methodof fluxions (the calculus, as we now call it), carried out most of his experimental work on

the nature and properties of light, and laid the foundations of the universal mechanics in

which he synthesized the terrestrial science of Galileo with the planetary theory of

Kepler. But he took many years to prepare these discoveries and inventions forpublication. Newton was very sensitive to criticism, and the equivocal reception of his

first communication to the Royal Society, on the nature of light, made him wary ofpublishing mere fragments of research. St) we find him holding on to his discoveries until

they could be worked up into massive treatises. ThePrincipia, the great work in which he

set out his mechanics and cosmology, did nw appear until 1687. The Opticks, most of the

experimental work for which had been done around 1666, was finally published only in1704.

In 1689 Newton took his seat in the House of Commons as a Member for Cambridge.This event marked a considerable change in his interests, and some historians have

suggested, in his character. He virtually abandoned scientific research from about this

time, and enjoyed the life of a senior administrator and public figure. He became Masterof Royal Mint and is said to have run it with exemplary efficiency. Throughout his life he

had taken an intense interest in theological matters. Even in old age he was still trying to

solve chronological problems in the dating of events recorded in the Old Testament. Hedied in 1727, having acquired a reputation in his own life-time that no other scientist was

ever quite to have again.

Early work on light and colour

Is colour a quality of light produced in a body, or is it a quality separated out of light by a

body? This seems a question of some profundity and its solution likely to be of greattechnical difficulty. The problem had a long history. Theodoric of Freibourg, whose

masterly solution of the difficulties of understanding the rainbow we have studied above,

was typical of medieval thinkers in generalizing a vaguely Aristotelian explanation. Hethought that light acquired its colour from the medium through which it passed. His

explanation is based upon the idea of pairs of contrary principles. A medium can be more

or less translucent. Near the surface a medium is more bounded than it is in its depths. A

mirror is perfectly bounded, and reflects all light, having no effect on colour. A


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transparent solid is unbounded, allowing light to penetrate deep into its interior. White

light is passed by a medium having a perfect balance of the four contraries. When a

medium is relatively bounded, that is near its surface, light is qualitatively changed so asto appear red. But when the medium is relatively opaque in its interior, the light is so

changed as to appear blue.

Fig. 30. The separation of rays of different 'refrangibility'. Newton, Opticks (1721 edn1,book 1, part 1, table iv, fig 18. S is the source of white light. In prism ABC the rays ofdifferent refrangibility are separated. The screens DE and de serve to separate

progressively purer colours.

This explanation could hardly be counted very satisfactory since the contraries seemed

rather more mysterious than the production of colours they were called upon to explain.A closer study of the way light was affected by transparent objects showed that the

colours had something to do with the way light was refracted when passing from one

medium such as glass to another, such as air. Descartes was the first to separate light ofpure colour using this effect. In Les Meteores of 1637 he describes an experiment which

he had performed in the course of studying the rainbow. The experimental arrangement isshown in Figure 20 ("Theodoric"). `When I covered one of these surfaces with a screen,'

says Descartes, `in which there was a small opening DE, I observed that the rays whichpass through this opening and are received on a white cloth or sheet of paper show all the

colours of the rainbow; and that the red always appears at F and the blue or violet at H.'

What relation did these coloured rays have to the light fron the sun which had fallen on

the prism? It was to the answer t~ this question that Newton's experiment was addressed.

Newton's systematic research programme

Newton's series of more and more successful versions of the basic experiment to bedescribed here was not original in conception, but it was to develop into a fairly exact

execution (For an account of the forerunners of Newton in the study of colour and

refraction see J. A. Lohne, Notes and Records of the Royal Society of London, 20, 1965,

pp. 125-39.) In his letter to the Royal Society of 1672, Newton tells of the puzzlement hefelt, when in an experiment of 1666, he noticed that the shat of the spectrum image cast

on a screen by passing light from round hole through a prism, was oblong, `with straight

sides' he says. Why should this be so? According to Lohne (see Further Reading),
http://iweb.tntech.edu/chem281-tf/Theodoric.htmhttp://iweb.tntech.edu/chem281-tf/Theodoric.htm


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Newton must have tidied up his description of this image somewhat, since the greater

intensity of the yellow component in the sun's light would have made the image rather

broader at that point in the spectrum.

In preparing a definitive account of the experiment for the Opticks, Newton describes

how he took pains to refine and sharpen the image. `By using a larger or smaller hole inthe window-shut [he] made the circular images larger or smaller at pleasure. The amount

of light could be increased by using a narrow oblong hole rather than a circular one,

keeping the ends of the spectrum image sharp.' Newton seems to have ignored oroverlooked diffraction effects of the use of a small hole as image, though these had been

noticed by his contemporaries.

The basic experiment, refined by the use of a lens to focus the image of the hole, wasquite simple: The spectrum is thrown on a piece of black paper in which there is a small

hole. When the hole coincides with the red part of the spectrum a beam of red light is

obtained, which can be refracted through a second prism. Similarly when the hole

coincides with the blue part of the spectrum a blue beam is separated out. It is the effectof the second prism that is the key. There are two results to be noticed. The resulting

image, whatever its colour, is quite circular, `which shows that the light is refractedwithout any dilatation of rays', since the shape of the hole is perfectly reproduced in the

image. But when a blue ray passes through the second prism it is more refracted than a

red ray. So the separation of the colours is a secondary effect. The underlying process is

the separation of `rays of different refrangibility'. In a letter to Lucas of 5 March 1677/8,Newton was at pains to emphasize the true result of the experiment. `. . . you think I

brought it to prove that rays of different colours are differently refrangible: whereas I

bring it to prove (without respect to colour) that light consist of rays differentlyrefrangible. What the colours of the rays differently refrangible are . . . belongs to after

enquiry . . .' (quoted by Lohne).

What is probably the last of Newton's many versions of the experiment is illustrated in

the engraving to be found in the Paris edition of the Opticks. It was drawn from a sketch

supplied by Newton himself (cf. Lohne, 1968).

So far Newton had achieved no more than a more

exact repetition of the cruder experiments of his

predecessors. Even the testing of monochromatic lightby passing it through a second prism had been

anticipated, albeit crudely, by J. M. Marci of

Kronland. Marci was a prominent physician inPrague. Though isolated from contacts with Western

scientists by the Catholic reaction in Bohemia in the

early seventeenth century, he did important work inastronomy, optics and medicine. But though he

succeeded in decomposing white light into coloured

beams, it was to be left to Newton successfully to

reconstitute the original beam.

Fig.31. The effect of using light

sources of different shapes.


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But to demonstrate that the phenomenon of colours in refracted light is caused by the

different refrangibility of rays already present in the white beam, and not by somemodification produced in the light by the glass of the optical apparatus, something more

is needed. Newton's original recombination experiment reported in the Letter of 1672

involved the use of a lens to bring about the confluence of the rays. The reactions ofmany of Newton's contemporaries to the experiment were tepid. Hooke objected that the

experiment does not show that the light, prior to refraction, should be thought of as a

collection of these different rays. They could have been produced in the process ofrefraction. However, in the Opticks Newton added another and very ingenious

recombination experiment to refute this kind of objection.

Fig.32. Decomposition, recomposition and decomposition of white light to the spectrum.

Newton, Opticks (1721 edn), book I, part II, table iv, fig.16. Rays refracted by prismABC are recombined optically by lens MN, and are reseparated by prism KIH.

By using a long, flat prism, Newton made the angle which separates the beams of

coloured light very small. By altering the angle of a screen arranged as in Figure 32,colours can be produced from what looks like white light. When the screen is at position

B, there is enough diffusion of light caused by dust particles in the air for the narrowly

separated coloured beams to be mixed again. By altering the angle of the screen to

position C the coloured beams are made to strike the screen at sufficiently separatedplaces for a spectrum to be seen. The distance WZ, separating the points of contact of the

red and blue beams with the screen in position C, is much greater than the distance XY

separating the images from the red and blue beams when the screen is in position B. Theonly feature of the arrangement which varies is the angle of the screen. The separation of

images is being brought about by manipulating something quite independent of the prism

which is producing the original, narrowly differentiated beams. Altering the angle of thescreen allows the differently coloured rays to be identified without the diffusion of light

from one beam to another which occurs when the images are very close together.


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Fig.33. Recombining colours without a lens.

To clinch the matter Newton undertook a much greater variety of optical manipulations

than Marci had attempted. Newton showed that once the colours had been properly

separated they were unaffected by any of his manipulations. Refraction and repeatedrefraction did not change the colour.

In a typical refraction experiment Newton illuminated an object with monochromatic

light, and then looked at it through a prism. If the passage of light from the object to the

eye through the prism had had any effect on the light then he should have seen somedifference in the colour of the thing when so observed. `But those illuminated with

homogeneous light appeared neither less distinct, nor otherwise coloured, than when

viewed with the naked eye.' Newton remarked that since the differences between the rays

might really be continuous, light could not be perfectly homogeneous, no matter howsharply focused. But the spread of colours in each apparently homogeneous ray is so

small that `change was not sensible, and therefore in experiments where sense is thejudge, the change ought not to be considered at all'. Truly homogeneous light cannot beproduced by refraction. Modern lasers which do produce perfectly coherent light depend

upon a different physical principle.

The final step was to examine a wide variety of substances, `paper, ashes, red lead, gold,

silver, copper, grass, blue flowers, violets, bubbles of water tinged with various colours,

peacock's feathers and such like . . .' Under red light, they all appeared red. Under blue

light they all looked blue, under green light, green and so on. Reflection, like refraction,has no effect on the colour of relatively homogeneous light.

The study of colour after Newton

But why are these results so readily and unambiguously obtained? Newton and Descartes

before him had supposed that in some way or another the motion of particles wasinvolved in the transmission of light. Newton considered the speed of the particles to be

the cause of our experiences o~ colour, while Descartes thought it had to do with their

rate o1 rotation. Eventually the problem was solved, at least relative to the knownphenomena, by Euler. About the year 1746 he gave precise mathematical form to another


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rival theory that had been proposed, notably by the Dutch physicist, Huyghens. Euler

showed that Newton's experimental results and many other phenomena could be

elegantly explained by assuming that light was propagated as a wave in an all-pervasivemedium, the luminiferous ether. Light was not to be thought of as a stream of particles,

but as vibration in an elastic solid. Colours corresponded to waves of different

wavelength. This explained why different colours were differentially refracted when theypassed from one medium to another. The colours were not produced in the medium, as

medieval physicists had thought, but at the boundary between media. Elegant though

Eider's solutions were, they too have to be modified under the pressure of still morerecondite discoveries about electromagnetic radiation of which light is only one rather

special kind.

In most of the experiments preceding Newton's study of colour, the subject underinvestigation lay ready to hand in the common experience of mankind. Falling bodies,

compressed gases, the rainbow and its accompanying drops of rain, even the developing

chick, are all within the range of our senses. In the conclusion Gilbert drew from

Norman's experiment a more subtle kind of being is proposed, something no humanobserver could ever experience. The orbis virtutis is the unobserved or `occult' cause of

observable magnetic effects. For all their apparent simplicity Newton's experiments oncolour also go beyond experience, though not so deeply as those of Norman and Gilbert.

Newton's refractions and screenings show that white light (which can never be perceived

by us as other than white) is `really' a mixture of coloured rays, which can be perceived

as they are, only when separated from all others by some accidental or humanmanipulation.

Newtons Experiments - light and prisms

This is Newtons famous experiment where a triangular prism is used to 'split' white lightinto the colours of the spectrum. In this apparatus we have a mains powered light box to

create parallel white light beams, two prisms, various slits to help collimate the beams as

well as small white projection screens. A solid wooden base is provided so thateverything can be easily aligned correctly.


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Experiment 1

We set up the light box (left hand side of above photo) to produce a slit of white light thatwill fall on the prism and about 15 cm away. In between the two set up a slit to make sure

that the light is as collimated as possible. 15 cm or so away from the prism on the other

side of the table set up a screen. Adjust (rotate) the prism until a spectrum of colours canbe seen on the screen. You may need to dim the room lighting so that it is clearly visable.

You can move the slit closer to the prism to improve the colour seperation. The lightfrom the light box should be composed of white rays all in parallel, but in practice there

will be some spread of angles coming from the box. If these unparallel rays go into the

prism they will each create a spectrum of their own and cause a jumbling of colours andso spoil the show. With the slit moved near to the prism it reduces the spread of rays so

they are all nearly parallel - it makes a lot of difference to the spectrum projected on the

screen but we do lose a little light in the process and so the spectrum is not quite sobright.


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The white light composed of many colours is going into the prism. Every colour will bebent (refracted) slightly differently by the materail properties of the glass. Red light is

bent least while the blue colours are bent greatest. As a result white light going in to theprism on one side emerges as a refracted spectrum of colours.

I used a powerful (2W) white LED in the light box. This is not a truly multicoloured

white light source like the rays from the Sun for example. As a result the spectrum is not

quite so perfect as you would see produced by small rain drops in the wonderful displayof a rainbow.

The prism orintation makes a lot of difference to the quality of the spectrum. The angle


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between the light rays coming in and the spectrum coming out of the prism can be seen

on the photo above and in practice adjust this for the best spectrum.

Experiment 2

Leaving the apparatus setup as in Experiment 1 move the screen slightly further away tosay 25 cm or so. Now midway between the prism and the screen insert the lens. The lens

will recombine the spectrum projecting a white light band / slit onto the screen. These

experiments therefore show that white light can be split into the spectrum but also thatthe spectrum can be reassembled into white light again !


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Experiment 3

During Newtons time there was some sceptism about what was precisely going on with

the white light and the prism glass. Some thought that the spectrum was directlyproduced by the glass rather than a seperation of colours brought about by the different

degrees of angle deflection (refraction). So Newton devised this simple experiment

(called 'Newton's Experimentum Crucis') to prove the case.

Set up as in Experiment 2 and introduce a second slit, second prism in place of the lens.

By adjusting the second slit one can chose a colour from the spectrum produced by thefirst prism (lets say the red light) and send this through the second prism. We dont see

another spectrum being produced from this red ray, instead we just see the red ray being

refracted by the second prism. We can do this for any other colour showing that thespectrum is not produced by some 'colouration' effect by the passage of the rays through

the glass prism.


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Newtons Color TheoryNewtons theory of color was that the suns light, or any other white light, was a mixture

of raysof light, each with its own refrangibility, by which he meant characteristic angle of

refraction in a

prism. Homogeneal light (which modern writers would call monochromatic) wouldalways

bend at its characteristic angle in a prism, but the differently refrangible rays that make

upwhite light are separated out into the rainbow by refracting to different degrees in the

prism.

This certainly is consistent with what we see when we put white light into a prism at just

theright angle: the spectrum forms at a predictable angle, with its red end bending least from

the

path the light was taking before entering the prism, and the violet, the least. In our lab, we

setup an optical bench where we used a bright line light to simulate the sun coming through

Newtons window shut, made the rays parallel with a collimating lens, shined it on aprism,

then focused the spectrum (using a lens) on a little screen.

However, at the time Newton was working on his Opticks experiments, there were

other


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explanations being proffered as to how the prism produced the rainbow. Hooke, for

example,

thought that white light was simple (presumably uniform, or pure) and that the prismsomehow

brought the colors into being by distorting the simple white light. Just by looking at the

spectrum, we cant distinguish which of these two explanations is closest to the truth.In addition to producing the spectrum on the optical bench, we simulated Newtons two-

prism

experiment, and directed the spectrum into a second prism, which turned the mixed lightwhite.

Newtons theory explains this result by saying that the second prism is recombining the

separate

rays so that they get to the observers eye mixed together in the way that we perceive aswhite.

Also, according to Newtons theory, we should only see the recombined light as white if

all the

parts of the spectrum are mixed together. When we inserted a little post in the spectrumso that

the part of it falling on the post was not entered into the mix at the second prism, therecombined

light was a color, not white. For example, if we blocked the middle part of the spectrum

(the

green) with the post, the recombined light was magenta! In fact, all of modern colormixing

theory could be demonstrated this way.

Newton said that homogeneal (monochromatic, in modern terms) light is not changedby

passage through a prism or lens, or by reflection off a mirror. For example, when we put

a greenfilter in the path of the light, the screen was dark where the red and blue light of the

spectrum

had been, and bright green just in the narrow middle. This is consistent with Newtonsclaim.

Newton said that white, black, and all the grays in between, were compounded of all the

colors

of the spectrum, mixed in due proportion. I would restate that as meaning in equalamounts.

When we placed a gray item in the spectrum, it reflected all the colors of the spectrum,

albeit lessbrightly than the white background. Even something we would call black reflected

uniformly

across the spectrum, just not very much. In fact, as our instructor showed us, a trueblack,

which does not reflect any of the light falling on it, is hard to come by. She made a small

hole in


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a piece of black construction paper, and used that as a window into a box. The hole was

much

blacker than the construction paper. Even the truly black object is on the same scale aswhite

and gray, since it reflects uniformly across the spectrum, except that the amount of

reflection forall parts of the spectrum is zero.

Newton thought that colors were a sensory experience, rather than a property of light.

Insofar asthe light had a physical property, it was the refrangibility of the ray, which we saw

remained

constant whether the color was made by a filter or by the prism. Lights of different

refrangibility tend to cause us to experience different colors. (So far as I know, Newtonmade no

claim as to how the light caused the sensation, or whether this had anything to do with

refrangibility.) The property of objects, on the other hand, which causes us to call them

colored,is their propensity to reflect this or that part of the spectrum more than another. This

claimpredicts that a colored object only reflects certain parts of the spectrum, and when placed

in the

spectrum, should either look the same color as the part of the spectrum shining on it, or

black. Ifyou can make a bright spectrum in a dark room, you can test this. We put variously

colored

objects into the different parts of the spectrum, and all of them either were the same coloras that

part of the spectrum shining on the white background, or were dark. For example, a

saturatedred object reflected the red part of the spectrum very strongly, but was dark everywhere

else.

Orange objects strongly reflected the red, orange, and yellow part of the spectrum, butwere dark

elsewhere, and so on.

Unweaving the RainbowFrom Wikipedia, the free encyclopedia

Jump to: navigation,searchFor the album by Frameshift, see Unweaving the Rainbow (album).

Unweaving the Rainbow
http://en.wikipedia.org/wiki/Unweaving_the_Rainbow#mw-head%23mw-headhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#mw-head%23mw-headhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#p-search%23p-searchhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow_(album)http://en.wikipedia.org/wiki/Unweaving_the_Rainbow#mw-head%23mw-headhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#p-search%23p-searchhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow_(album)


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Author(s) Richard Dawkins

Subject(s) Evolutionary biology

Publisher Boston : Houghton Mifflin

Publication date 1998

Pages 336

ISBN 0-618-05673-4

OCLC Number 45155530

Preceded by Climbing Mount Improbable

Followed by A Devil's Chaplain

Unweaving the Rainbow(subtitled "Science, Delusion and the Appetite for Wonder") is

a 1998 book byRichard Dawkins, discussing the relationship between science and the

arts from the perspective of a scientist.

Dawkins addresses the misperception that science and art are at odds. Driven by theresponses to his books The Selfish Gene and The Blind Watchmakerwherein readers

resented his naturalistic world view, seeing it as depriving life of meaning, Dawkins felt

the need to explain that, as a scientist, he saw the world as full of wonders and a source ofpleasure. This pleasure was not in spite of, but rather because he does not assume as

cause the inexplicable actions of a deitybut rather the understandable laws of nature.
http://en.wikipedia.org/wiki/Richard_Dawkinshttp://en.wikipedia.org/wiki/Evolutionary_biologyhttp://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/0-618-05673-4http://en.wikipedia.org/wiki/Online_Computer_Library_Centerhttp://worldcat.org/oclc/45155530http://en.wikipedia.org/wiki/Climbing_Mount_Improbablehttp://en.wikipedia.org/wiki/A_Devil's_Chaplainhttp://en.wikipedia.org/wiki/Richard_Dawkinshttp://en.wikipedia.org/wiki/Richard_Dawkinshttp://en.wikipedia.org/wiki/Sciencehttp://en.wikipedia.org/wiki/The_artshttp://en.wikipedia.org/wiki/The_artshttp://en.wikipedia.org/wiki/The_Selfish_Genehttp://en.wikipedia.org/wiki/The_Blind_Watchmakerhttp://en.wikipedia.org/wiki/Wonder_(emotion)http://en.wikipedia.org/wiki/Deityhttp://en.wikipedia.org/wiki/Deityhttp://en.wikipedia.org/wiki/File:Unweaving_the_Rainbow.jpghttp://en.wikipedia.org/wiki/Richard_Dawkinshttp://en.wikipedia.org/wiki/Evolutionary_biologyhttp://en.wikipedia.org/wiki/International_Standard_Book_Numberhttp://en.wikipedia.org/wiki/Special:BookSources/0-618-05673-4http://en.wikipedia.org/wiki/Online_Computer_Library_Centerhttp://worldcat.org/oclc/45155530http://en.wikipedia.org/wiki/Climbing_Mount_Improbablehttp://en.wikipedia.org/wiki/A_Devil's_Chaplainhttp://en.wikipedia.org/wiki/Richard_Dawkinshttp://en.wikipedia.org/wiki/Sciencehttp://en.wikipedia.org/wiki/The_artshttp://en.wikipedia.org/wiki/The_artshttp://en.wikipedia.org/wiki/The_Selfish_Genehttp://en.wikipedia.org/wiki/The_Blind_Watchmakerhttp://en.wikipedia.org/wiki/Wonder_(emotion)http://en.wikipedia.org/wiki/Deity


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His starting point isJohn Keats' well-known, light-hearted accusation that Isaac Newton

destroyed the poetry of the rainbow by 'reducing it to the prismatic colours.' [1]

(Incidentally, Newton did no such thing: it was Theodoric of Freiberg who discoveredrainbows were prismatic. Newton's famous discovery with prisms was recombination of a

spectrum back into white light.) The agenda of the book is to show the reader that science

does not destroy, but rather discovers poetry in the patterns of nature.

Contents

[hide]

1 Summary of the arguments

o 1.1 Preface

o 1.2 The anaesthetic of familiarity

1.2.1 Opening lines

1.2.2 Summaryo 1.3 Drawing room of dukeso 1.4 Barcodes in the stars

o 1.5 Barcodes on the air

o 1.6 Barcodes at the bar

o 1.7 Hoodwink'd with faery fancy

o 1.8 Unweaving the uncanny

o 1.9 Huge cloudy symbols of a high romance

o 1.10 The selfish cooperator

o 1.11 The genetic Book of the Dead

o 1.12 Reweaving the world

o 1.13 The balloon of the mind

1.13.1 Conclusion 2 Petwhac

3 Notes

4 External links

[edit] Summary of the arguments

The following summary of the book's arguments in favour of science does not attempt toreproduce the actual explanations of scientific phenomena (how DNA works,petwhac,

etc.), which in fact form most of the text.

[edit] PrefaceIt is of little concern whether or not science can prove that the ultimate fate of the cosmos

lackspurpose: we live our lives regardless at a "human" level, according to ambitions and

perceptionswhich come more naturally. Therefore, science should not be feared as a sortof cosmological wet blanket. In fact, those in search of beauty orpoetry in their
http://en.wikipedia.org/wiki/John_Keatshttp://en.wikipedia.org/wiki/John_Keatshttp://en.wikipedia.org/wiki/John_Keatshttp://en.wikipedia.org/wiki/Isaac_Newtonhttp://en.wikipedia.org/wiki/Rainbowhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Theodoric_of_Freiberghttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#%23http://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Summary_of_the_arguments%23Summary_of_the_argumentshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Preface%23Prefacehttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#The_anaesthetic_of_familiarity%23The_anaesthetic_of_familiarityhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Opening_lines%23Opening_lineshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Summary%23Summaryhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Drawing_room_of_dukes%23Drawing_room_of_dukeshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Barcodes_in_the_stars%23Barcodes_in_the_starshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Barcodes_on_the_air%23Barcodes_on_the_airhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Barcodes_at_the_bar%23Barcodes_at_the_barhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Hoodwink.27d_with_faery_fancy%23Hoodwink.27d_with_faery_fancyhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Unweaving_the_uncanny%23Unweaving_the_uncannyhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Huge_cloudy_symbols_of_a_high_romance%23Huge_cloudy_symbols_of_a_high_romancehttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#The_selfish_cooperator%23The_selfish_cooperatorhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#The_genetic_Book_of_the_Dead%23The_genetic_Book_of_the_Deadhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Reweaving_the_world%23Reweaving_the_worldhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#The_balloon_of_the_mind%23The_balloon_of_the_mindhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Conclusion%23Conclusionhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Petwhac%23Petwhachttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Notes%23Noteshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#External_links%23External_linkshttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=1http://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Petwhac%23Petwhachttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Petwhac%23Petwhachttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=2http://en.wikipedia.org/wiki/Cosmoshttp://en.wikipedia.org/wiki/Meaning_of_lifehttp://en.wikipedia.org/wiki/Ambitionhttp://en.wikipedia.org/wiki/Perceptionhttp://en.wikipedia.org/wiki/Perceptionhttp://en.wikipedia.org/wiki/Poetryhttp://en.wikipedia.org/wiki/John_Keatshttp://en.wikipedia.org/wiki/Isaac_Newtonhttp://en.wikipedia.org/wiki/Rainbowhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Theodoric_of_Freiberghttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#%23http://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Summary_of_the_arguments%23Summary_of_the_argumentshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Preface%23Prefacehttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#The_anaesthetic_of_familiarity%23The_anaesthetic_of_familiarityhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Opening_lines%23Opening_lineshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Summary%23Summaryhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Drawing_room_of_dukes%23Drawing_room_of_dukeshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Barcodes_in_the_stars%23Barcodes_in_the_starshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Barcodes_on_the_air%23Barcodes_on_the_airhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Barcodes_at_the_bar%23Barcodes_at_the_barhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Hoodwink.27d_with_faery_fancy%23Hoodwink.27d_with_faery_fancyhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Unweaving_the_uncanny%23Unweaving_the_uncannyhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Huge_cloudy_symbols_of_a_high_romance%23Huge_cloudy_symbols_of_a_high_romancehttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#The_selfish_cooperator%23The_selfish_cooperatorhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#The_genetic_Book_of_the_Dead%23The_genetic_Book_of_the_Deadhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Reweaving_the_world%23Reweaving_the_worldhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#The_balloon_of_the_mind%23The_balloon_of_the_mindhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Conclusion%23Conclusionhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Petwhac%23Petwhachttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Notes%23Noteshttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#External_links%23External_linkshttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=1http://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/Unweaving_the_Rainbow#Petwhac%23Petwhachttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=2http://en.wikipedia.org/wiki/Cosmoshttp://en.wikipedia.org/wiki/Meaning_of_lifehttp://en.wikipedia.org/wiki/Ambitionhttp://en.wikipedia.org/wiki/Perceptionhttp://en.wikipedia.org/wiki/Poetry


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cosmology need not turn to theparanormal or even necessarily restrict themselves to the

mysterious: science itself, the business of unravelling mysteries, is beautiful and poetic.

(The rest of the preface sketches an outline of the book, makes acknowledgements, etc.)

[edit] The anaesthetic of familiarity

[edit] Opening lines

"We are going to die, and that makes us the lucky ones. Most people are never going to

die because they are never going to be born. The potential people who could have beenhere in my place but who will in fact never see the light of day outnumber the sand grains

of Arabia. Certainly those unborn ghosts include greater poets than Keats, scientists

greater than Newton. We know this because the set of possible people allowed by ourDNA so massively outnumbers the set of actual people. In the teeth of these stupefying

odds it is you and I, in our ordinariness, that are here. We privileged few, who won the

lottery of birth against all odds, how dare we whine at our inevitable return to that priorstate from which the vast majority have never stirred?"

Richard Dawkins has stated on several occasions that these lines should be read at hisfuneral.

[edit] Summary

The first chapter describes several ways in which the universe appears beautiful and

poetic when viewed scientifically. However, it first introduces an additional reason to

embrace science. Time and spaceare vast, so theprobabilitythat the reader came to be

alive here and now, as opposed to another time or place, was slim. More important, theprobability that the reader came to be alive at all were even slimmer: the correct structure

ofatoms had to align in the universe. Given how special these circumstances are, the

"noble" thing to do is employ the allotted several decades of human life towardsunderstanding that universe. Rather than simply feeling connected with nature, one

should rise above this "anaesthetic of familiarity" and observe the universe scientifically.

[edit] Drawing room of dukesThis chapter describes a third reason to embrace science (the first two being beauty and

duty): improving one's performance in the arts. Science is often presented publicly in atranslated format, "dumbed down" to fit the language and existing ideas of non-scientists.

This offers a disservice to the public, who are capable of appreciating the beauty of theuniverse as deeply as a scientist can. The successful communication of unadulteratedscience enhances, not confuses, the arts; after all, poets (Dawkins' synonym for artists

see page 24) and scientists are motivated by a similar spirit of wonder. We should

therefore battle thestereotype that science is difficult, uncool, and not useful for thecommon person.
http://en.wikipedia.org/wiki/Cosmologyhttp://en.wikipedia.org/wiki/Paranormalhttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=4http://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=5http://en.wikipedia.org/wiki/Space-timehttp://en.wikipedia.org/wiki/Space-timehttp://en.wikipedia.org/wiki/Probabilityhttp://en.wikipedia.org/wiki/Probabilityhttp://en.wikipedia.org/wiki/Probabilityhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Anaesthetichttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=6http://en.wikipedia.org/wiki/The_artshttp://en.wikipedia.org/wiki/Stereotypehttp://en.wikipedia.org/wiki/Stereotypehttp://en.wikipedia.org/wiki/Cosmologyhttp://en.wikipedia.org/wiki/Paranormalhttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=4http://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=5http://en.wikipedia.org/wiki/Space-timehttp://en.wikipedia.org/wiki/Probabilityhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Anaesthetichttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=6http://en.wikipedia.org/wiki/The_artshttp://en.wikipedia.org/wiki/Stereotype


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[edit] Barcodes in the starsStudying a phenomenon, such as a flower, cannot detract from its beauty. First, some

scientists, such as Feynman, are able to appreciate theaesthetics of the flower whileengaged in their study. Second, the mysteries which science unfolds lead to new and

more exciting mysteries; for example,botany's findings might lead us to wonder about

the workings of a fly's consciousness. This effect of multiplying mysteries should satisfyeven those who think that scientific understanding is at odds with aesthetics, e.g. people

who agree with Einstein that "the most beautiful thing we can experience is the

mysterious". (For evidence, the rest of this chapter discusses the fascinating science andbeautiful new mysteries which followed in the wake of Newton's "unweaving" of the

rainbow, q.e. his explanation of theprismatic effects of moist air.)

[edit] Barcodes on the airThis chapter offers more evidence that science is fun and poetic, by exploring sound

waves,birdsong, and low-frequency phenomena such aspendula and periodic mass

extinctions.

[edit] Barcodes at the barA fourth reason to embrace science is that it can help deliver justice in a court of law, viaDNA fingerprinting or even via simple statistical reasoning. Everyone should learn the

scientist's art of probability assessment, to make better decisions.

[edit] Hoodwink'd with faery fancyThis chapter explores what Dawkins considers to be fallacies inastrology,religion,

magic, and extraterrestrial visitations. Credulity andHume's criterion are also discussed.

[edit] Unweaving the uncannyAmazing coincidences are much more common than we may think, and sometimes, whenover-interpreted, they lead to faulty conclusions. Statistical significance tests can help

determine which patterns are meaningful.

[edit] Huge cloudy symbols of a high romanceUnlike "magisterial poetry" (where metaphors and pretty language are used to describe

the familiar), "pupillary poetry" uses poetic imagery to assist a scientist's thinking about

the exotic (e.g. consider "being" an electron temporarily). Although it is useful, someauthors take pupillary poetry too far, and, "drunkon metaphor", they produce "bad

science"; i.e. postulate faulty theories. This is powered by humanity's natural tendency to

look for representations.

[edit] The selfish cooperatorGenes compete with each other, but this occurs within the context of collaboration, as isshown with examples involving mitochondria,bacteria, and termites. Two types of

collaboration are co-adaptation (tailoring simultaneously the different parts of an
http://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=7http://en.wikipedia.org/wiki/Feynmanhttp://en.wikipedia.org/wiki/Aestheticshttp://en.wikipedia.org/wiki/Aestheticshttp://en.wikipedia.org/wiki/Botanyhttp://en.wikipedia.org/wiki/Flyhttp://en.wikipedia.org/wiki/Consciousnesshttp://en.wikipedia.org/wiki/Aestheticshttp://en.wikipedia.org/wiki/Prism_(optics)http://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=8http://en.wikipedia.org/wiki/Sound_wavehttp://en.wikipedia.org/wiki/Sound_wavehttp://en.wikipedia.org/wiki/Bird_vocalizationhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Pendulahttp://en.wikipedia.org/wiki/Mass_extinctionhttp://en.wikipedia.org/wiki/Mass_extinctionhttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=9http://en.wikipedia.org/wiki/DNA_fingerprintinghttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=10http://en.wikipedia.org/wiki/Astrologyhttp://en.wikipedia.org/wiki/Astrologyhttp://en.wikipedia.org/wiki/Astrologyhttp://en.wikipedia.org/wiki/Religionhttp://en.wikipedia.org/wiki/Magic_(paranormal)http://en.wikipedia.org/wiki/Magic_(paranormal)http://en.wikipedia.org/wiki/Extraterrestrial_hypothesishttp://en.wikipedia.org/wiki/David_Humehttp://en.wikipedia.org/wiki/David_Humehttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=11http://en.wikipedia.org/wiki/Statistical_significance_testhttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=12http://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Drunkhttp://en.wikipedia.org/wiki/Drunkhttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=13http://en.wikipedia.org/wiki/Genehttp://en.wikipedia.org/wiki/Mitochondriahttp://en.wikipedia.org/wiki/Bacteriahttp://en.wikipedia.org/wiki/Termitehttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=7http://en.wikipedia.org/wiki/Feynmanhttp://en.wikipedia.org/wiki/Aestheticshttp://en.wikipedia.org/wiki/Botanyhttp://en.wikipedia.org/wiki/Flyhttp://en.wikipedia.org/wiki/Consciousnesshttp://en.wikipedia.org/wiki/Aestheticshttp://en.wikipedia.org/wiki/Prism_(optics)http://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=8http://en.wikipedia.org/wiki/Sound_wavehttp://en.wikipedia.org/wiki/Sound_wavehttp://en.wikipedia.org/wiki/Bird_vocalizationhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Pendulahttp://en.wikipedia.org/wiki/Mass_extinctionhttp://en.wikipedia.org/wiki/Mass_extinctionhttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=9http://en.wikipedia.org/wiki/DNA_fingerprintinghttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=10http://en.wikipedia.org/wiki/Astrologyhttp://en.wikipedia.org/wiki/Religionhttp://en.wikipedia.org/wiki/Magic_(paranormal)http://en.wikipedia.org/wiki/Extraterrestrial_hypothesishttp://en.wikipedia.org/wiki/David_Humehttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=11http://en.wikipedia.org/wiki/Statistical_significance_testhttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=12http://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Drunkhttp://en.wikipedia.org/w/index.php?title=Unweaving_the_Rainbow&action=edit&section=13http://en.wikipedia.org/wiki/Genehttp://en.wikipedia.org/wiki/Mitochondriahttp://en.wikipedia.org/wiki/Bacteriahttp://en.wikipedia.org/wiki/Termite


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The first is explained by the fact that the clock had a mechanical defect which made it

stop when tilted off the horizontal, which is what a nurse did to read the time of death in

poor lighting conditions. The matter of the watches, in Dawkins' own words, is explainedthus

If somebody's watch stopped three weeks after the spell was cast, even the mostcredulous would prefer to put it down to chance. We need to decide how large a delay

would have been judged by the audience as sufficiently simultaneous with the psychic'sannouncement to impress. About five minutes is certainly safe, especially since he can

keep talking to each caller for a few minutes before the next call ceases to seem roughly

simultaneous. There are about 100,000 five-minute periods in a year. The probabilitythat any given watch, say mine, will stop in a designated five-minute period is about 1 in

100,000. Low odds, but there are 10 million people watching the show. If only half of

them are wearing watches, we could expect about 25 of those watches to stop in anygiven minute. If only a quarter of these ring in to the studio, that is 6 calls, more than

enough to dumbfound a nave audience. Especially when you add in the calls from

people whose watches stopped the day before, people whose watches didn't stop butwhose grandfather clocks did, people who died of heart attacks and their bereaved

relatives phoned in to say that their 'ticker' gave out, and so on.

Dawkins defends his choice of the word "population" by writing "Population may seem

an odd word, but it is the correct statistical term.", adding "I won't keep using capitalletters because they stand so unattractively on the page."

Scientific history

The classical GreekscholarAristotle(384322 BC) was first to devote serious attention

to the rainbow. According to Raymond L. Lee and Alistair B. Fraser, "Despite its many

flaws and its appeal to Pythagorean numerology, Aristotle's qualitative explanationshowed an inventiveness and relative consistency that was unmatched for centuries. After

Aristotle's death, much rainbow theory consisted of reaction to his work, although not all

of this was uncritical."[15]

In theNaturales Quaestiones(ca. 65 AD), Senecadevotes a whole book to rainbows,

heaping up a number of observations and hypotheses. He notices that rainbows appearalways opposite to the sun, that they appear in water sprayed by a rower or even in the

water spat by a laundereron dresses; he even speaks of rainbows produced by small rods

(virgulae) of glass, anticipatingNewton's experiences with prisms. He takes into accounttwo theories: one, that the rainbow is produced by the sun reflecting in each water-drop,

the other, that it is produced by the sun reflected in a cloud shaped like a concave mirror.He favors the latter theory. He observes other phenomena related with rainbows: themysterious "virgae" (rods) and theparhelia.

The Persian physicist andpolymath,Ibn al-Haytham (Alhazen; 9651039), attempted toprovide a scientific explanation for the rainbow phenomenon. In his Maqala fi al-Hala

wa Qaws Quzah (On the Rainbow and Halo), he "explained the formation of rainbow as

an image, which forms at a concave mirror. If the rays of light coming from a farther
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light source reflect to any point on axis of the concave mirror, they form concentric

circles in that point. When it is supposed that the sun as a farther light source, the eye of

viewer as a point on the axis of mirror and a cloud as a reflecting surface, then it can beobserved the concentric circles are forming on the axis."[16] He was not able to verify this

because his theory that "light from the sun is reflected by a cloud before reaching the

eye" did not allow for a possibleexperimentalverification.[17]

This explanation was laterrepeated by Averroes,[16]and, though incorrect, provided the groundwork for the correct

explanations later given byKaml al-Dn al-Fris(1267ca. 1319/1320) andTheodoric

of Freiberg (c.12501310).[18] Ibn al-Haytham supported the Aristotelian views that therainbow is caused by reflection alone and that its colours are not real like object colours.[19]

Ibn al-Haytham's contemporary, the Persian philosopherand polymath Ibn Sn

(Avicenna; 9801037), provided an alternative explanation, writing "that the bow is not

formed in the dark cloud but rather in the very thin mist lying between the cloud and the

sun or observer. The cloud, he thought, serves simply as the background of this thin

substance, much as a quicksilver lining is placed upon the rear surface of the glass in amirror. Ibn Sn would change the place not only of the bow, but also of the colour

formation, holding the iridescence to be merely a subjective sensation in the eye." [20] Thisexplanation, however, was also incorrect.[16] Ibn Sn's account accepts many of

Aristotle's arguments on the rainbow.[19]

In Song Dynasty China (9601279), a polymathic scholar-official named Shen Kuo

(10311095) hypothesizedas a certain Sun Sikong (10151076) did before himthat

rainbows were formed by a phenomenon of sunlight encountering droplets of rain in theair.[21]Paul Dong writes that Shen's explanation of the rainbow as a phenomenon of

atmospheric refraction"is basically in accord with modern scientific principles."[22]

The Persian astronomer, Qutb al-Din al-Shirazi(12361311), gave a fairly accurate

explanation for the rainbow phenomenon. This was elaborated on by his student,Kaml

al-Dn al-Fris(12601320), who gave a more mathematically satisfactory explanationof the rainbow. He "proposed a model where the ray of light from the sun was refracted

twice by a water droplet, one or more reflections occurring between the two refractions."

An experiment with a water-filled glass sphere was conducted and al-Farisi showed theadditional refractions due to the glass could be ignored in his model .[17]As he noted in his

Kitab Tanqih al-Manazir(The Revision of the Optics), al-Farisi used a large clear vessel

of glass in the shape of a sphere, which was filled with water, in order to have an

experimental large-scale model of a rain drop. He then placed this model within a camera

obscura that has a controlled aperture for the introduction of light. He projected light untothe sphere and ultimately deduced through several trials and detailed observations of

reflections and refractions of light that the colours of the rainbow are phenomena of thedecomposition of light. His research had resonances with the studies of his contemporary

Theodoric of Freiberg (without any contacts between them; even though they both relied

on Aristotle's and Ibn al-Haytham's legacy), and later with the experiments ofDescartesandNewton in dioptrics (for instance, Newton conducted a similar experiment at Trinity

College, though using a prism rather than a sphere).[23][24][25][26][verification needed][clarification needed]
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ia.org/wiki/Rainbow#cite_note-22%23cite_note-22http://en.wikipedia.org/wiki/Rainbow#cite_note-23%23cite_note-23http://en.wikipedia.org/wiki/Rainbow#cite_note-24%23cite_note-24http://en.wikipedia.org/wiki/Rainbow#cite_note-25%23cite_note-25http://en.wikipedia.org/wiki/Wikipedia:Verifiabilityhttp://en.wikipedia.org/wiki/Wikipedia:Please_clarify


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In Europe, Ibn al-Haytham'sBook of Optics was translated into Latin and studied by

Robert Grosseteste. His work on light was continued by Roger Bacon, who wrote in hisOpus Majus of 1268 about experiments with light shining through crystals and waterdroplets showing the colours of the rainbow.[27] In addition, Bacon was the first to

calculate the angular size of the rainbow. He stated that the rainbow summit can not

appear higher than 42 above the horizon.[28]

Theodoric of Freiberg is known to havegiven an accurate theoretical explanation of both the primary and secondary rainbows in

1307. He explained the primary rainbow, noting that "when sunlight falls on individual

drops of moisture, the rays undergo two refractions (upon ingress and egress) and onereflection (at the back of the drop) before transmission into the eye of the observer".[29]

He explained the secondary rainbow through a similar analysis involving two refractions

and two reflections.

Ren Descartes' sketch of how primary and secondary rainbows are formed

Descartes' 1637 treatise,Discourse on Method, further advanced this explanation.

Knowing that the size of raindrops did not appear to affect the observed rainbow, he

experimented with passing rays of light through a large glass sphere filled with water. Bymeasuring the angles that the rays emerged, he concluded that the primary bow wascaused by a single internal reflection inside the raindrop and that a secondary bow could

be caused by two internal reflections. He supported this conclusion with a derivation of

the law ofrefraction (subsequently to, but independently of, Snell) and correctlycalculated the angles for both bows. His explanation of the colours, however, was based

on a mechanical version of the traditional theory that colours were produced by a

modification of white light.[30][31]

Isaac Newton demonstrated that white light was composed of the light of all the colours

of the rainbow, which a glassprism could separate into the full spectrum of colours,

rejecting the theory that the colours were produced by a modification of white light. Healso showed that red light is refracted less than blue light, which led to the first scientific

explanation of the major features of the rainbow.[32]Newton's corpuscular theory of lightwas unable to explain supernumerary rainbows, and a satisfactory explanation was not

found until Thomas Young realised that light behaves as a wave under certain conditions,

and can interfere with itself.
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Young's work was refined in the 1820s byGeorge Biddell Airy, who explained the

dependence of the strength of the colours of the rainbow on the size of the water droplets.

Modern physical descriptions of the rainbow are based on Mie scattering, work publishedby Gustav Miein 1908. Advances in computational methods and optical theory continue

to lead to a fuller understanding of rainbows. For example,Nussenzveig provides a

modern overview.[33]

Spectrum

A rainbow spans a continuous spectrum of coloursthere are no "bands". The apparentdiscreteness is an artefact of thephotopigmentsin the human eye and of the neural

processing of ourphotoreceptoroutputs in the brain. Because the peak response of human

colour receptors varies from person to person, different individuals will see slightlydifferent colours, and persons with colour blindness will see a smaller set of colours.

However, the seven colours listed below are thought to be representative of how humans

everywhere,[2] with normal colour vision, see the rainbow.

Newton originally (1672) named onlyfive primary colours: red,yellow, green,blueandviolet. Later he included orange and indigo, giving seven colours by analogy to thenumber of notes in a musical scale. [3]

Red Orange Yellow Green Blue Indigo Violet

Rainbow

A rainbow is an optical and meteorological phenomenon that causes a spectrumoflight

to appear in the sky when the Sun shines on to droplets of moisture in the Earth'satmosphere. It takes the form of amulticolouredarc. Rainbows caused by sunlight

always appear in the section of sky directly opposite the sun.

In a so-called "primary rainbow" (the lowest, and also normally the brightest rainbow)

the arc of a rainbow shows red on the outer (or upper) part of the arc, and violet on the

inner section. This rainbow is caused by light being refracted then reflected once indroplets of water. In a double rainbow, a second arc may be seen above and outside the

primary arc, and has the order of its colours reversed (red faces inward toward the otherrainbow, in both rainbows). This second rainbow is caused by light reflecting twiceinside water droplets. The region between a double rainbow is dark, and is known as

"Alexander's band" or "Alexander's dark band". The reason for this dark band is that,

while light below the primary rainbow comes from droplet reflection, and light above the

upper (secondary) rainbow also comes from droplet reflection, there is no mechanism forthe region between a double rainbow to show any light reflected from water drops.
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It is impossible for an observer to manoeuvre to see any rainbow from water droplets at

any angle other than the customary one (which is 42 degrees from the direction opposite

the Sun). Even if an observer sees another observer who seems "under" or "at the end" ofa rainbow, the second observer will see a different rainbow further off-yet, at the same

angle as seen by the first observer. Thus, a "rainbow" is not a physical object, and cannot

be physically approached.

A rainbow spans a continuous spectrum of colours; the distinct bands (including the

number of bands) are an artefact of human colour vision, and no banding of any type isseen in a black-and-white photo of a rainbow (only a smooth gradation of intensity to a

maximum, then fading to a minimum at the other side of the arc). For colours seen by a

normal human eye, the most commonly cited and remembered sequence, in English, isNewton's sevenfold red, orange, yellow, green, blue,indigo and violet (popularly

memorized by mnemonics like Roy G. Biv). However, colour-blind persons will see

fewer colours.

Rainbows can be caused by many forms of airborne water. These include not only rain,but also mist, spray, and airborne dew.
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rainbow formation

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