   #copyright

Rainbow

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Full featured rainbow in Wrangell-St. Elias National Park, Alaska.
   Enlarge
   Full featured rainbow in Wrangell-St. Elias National Park, Alaska.
   Rainbow in the spray and mist of the waterfall at Takakkaw Falls,
   Canada
   Enlarge
   Rainbow in the spray and mist of the waterfall at Takakkaw Falls,
   Canada

   A rainbow is an optical and meteorological phenomenon that causes a
   nearly continuous spectrum of light to appear in the sky when the Sun
   shines onto droplets of moisture in the Earth's atmosphere. It takes
   the form of a multicoloured arc, with red on the outside and violet on
   the inside. A double rainbow includes a second, fainter, arc with
   colors in the opposite order.

   Even though a rainbow spans a continuous spectrum of colours,
   traditionally the full sequence of colours is most commonly cited as
   red, orange, yellow, green, blue, indigo and violet. It is commonly
   thought that indigo was included due to the different religious
   connotations of the numbers six and seven at the time of Isaac Newton's
   work on light, despite its lack of scientific significance and the poor
   ability of humans to distinguish colours in the blue portion of the
   visual spectrum.

Scientific explanation

   The rainbow effect can be observed whenever there are water drops in
   the air and sunlight shining from behind the observer at a low altitude
   or angle. The most spectacular rainbow displays when half of the sky is
   still dark with draining clouds and the observer is at a spot with
   clear sky overhead. The rainbow effect is also commonly seen near
   waterfalls or fountains. Rainbow fringes can sometimes be seen at the
   edges of backlit clouds and as vertical bands in distant rain or virga.
   The effect can also be artificially created by dispersing water
   droplets into the air during a sunny day.

   In a very few cases, a moonbow, or night-time rainbow, can be seen on
   strongly moonlit nights. As human visual perception for colour in low
   light is poor, moonbows are most often perceived to be white.
   Rainbows may also appear in the spray of a water fountain
   Enlarge
   Rainbows may also appear in the spray of a water fountain

   The rainbow's appearance is caused by dispersion of sunlight as it is
   refracted by (approximately spherical) raindrops. The light is first
   refracted as it enters the surface of the raindrop, reflected off the
   back of the drop, and again refracted as it leaves the drop. The
   overall effect is that the incoming light is reflected back over a wide
   range of angles, with the most intense light at an angle of about
   40°–42°. This angle is independent of the size of the drop, but does
   depend on its refractive index. As seawater has a higher refractive
   index than rain water, the radius of a 'rain'bow in a sea spray is
   smaller than a true rainbow. This is visible to the naked eye by a
   misalignment of these bows .

   Since the water is dispersive, the amount that the sunlight is bent
   depends upon the wavelength, and hence colour, of the light's
   constituent parts. Blue light is refracted at a greater angle than red
   light, but because the area of the back of the droplet has a focal
   point inside the droplet, the spectrum crosses itself, and therefore
   the red light appears higher in the sky, and forms the outer colour of
   the rainbow. Contrary to popular belief, the light at the back of the
   raindrop does not undergo total internal reflection; however, light
   that emerges from the back of the raindrop does not create a rainbow
   between the observer and the Sun. The spectra emitted from the back of
   the raindrop do not have a maximum of intensity, as the other visible
   rainbows do, and thus the colours blend together and do not form a
   rainbow.

   A raindrop has been placed inside a half sphere so that the scattered
   light can be seen. People seeing this one raindrop from different
   directions would preceive it as either red, yellow, green, blue or
   violet (or colourless).
   Enlarge
   A raindrop has been placed inside a half sphere so that the scattered
   light can be seen. People seeing this one raindrop from different
   directions would preceive it as either red, yellow, green, blue or
   violet (or colourless).
   Light rays enter a raindrop from one direction (typically a straight
   line from the Sun), reflect off the back of the raindrop, and fan out
   as they leave the raindrop. The light leaving the rainbow is spread
   over a wide angle, with a maximum intensity of 40.6°–42°.
   Enlarge
   Light rays enter a raindrop from one direction (typically a straight
   line from the Sun), reflect off the back of the raindrop, and fan out
   as they leave the raindrop. The light leaving the rainbow is spread
   over a wide angle, with a maximum intensity of 40.6°–42°.
   White light separates into different colours (wavelengths) on entering
   the raindrop because red light is refracted by a lesser angle than blue
   light. On leaving the raindrop, the red rays have turned through a
   smaller angle than the blue rays, producing a rainbow.
   Enlarge
   White light separates into different colours ( wavelengths) on entering
   the raindrop because red light is refracted by a lesser angle than blue
   light. On leaving the raindrop, the red rays have turned through a
   smaller angle than the blue rays, producing a rainbow.

   Some rainbows appear to be exceptionally close. This one is formed by a
   waterfall and is less than 20 feet away from the camera.
   Enlarge
   Some rainbows appear to be exceptionally close. This one is formed by a
   waterfall and is less than 20 feet away from the camera.

   A rainbow does not actually exist at a location in the sky, but rather
   is an optical phenomenon whose apparent position depends on the
   observer's location. All raindrops refract and reflect the sunlight in
   the same way, but only the light from some raindrops reaches the
   observer's eye. These raindrops are perceived to constitute the rainbow
   by that observer. The position of a rainbow in the sky is always in the
   opposite direction of the Sun with respect to the observer, and the
   interior is always slightly brighter than the exterior. The bow is
   centred on the shadow of the observer's head, or more exactly at the
   antisolar point (which is below the horizon during the daytime),
   appearing at an angle of approximately 40°–42° to the line between the
   observer's head and its shadow. As a result, if the Sun is higher than
   42°, then the rainbow is below the horizon and cannot be seen as there
   are usually not enough raindrops between the horizon (that is: eye
   height) and the ground, to contribute. One exception is when the
   observer is at the top of a mountain or a similar vantage point, for
   example an aeroplane (see below). Another exception occurs when the
   rainbow is produced by a garden sprinkler. In this case to get
   sufficient drops they must be very small.
   A portion of a 360 degree rainbow, seen from an airplane.
   Enlarge
   A portion of a 360 degree rainbow, seen from an airplane.

   It is difficult to photograph the complete arc of a rainbow, which
   would require an angle of view of 84°. For a 35 mm camera, a lens with
   a focal length of 19  mm or less would be required, whilst most
   photographers are only likely to have a 28 mm wide-angle lens. From an
   aeroplane, one has the opportunity to see the whole circle of the
   rainbow, with the plane's shadow in the centre. This phenomenon can be
   confused with the glory, but a glory is usually much smaller, covering
   only 5°–20°, as opposed to over 80° for a full circle rainbow.

Variations

   Occasionally, a second, dimmer secondary rainbow is seen outside the
   primary bow. Secondary rainbows are caused by a double reflection of
   sunlight inside the raindrops, and appear at an angle of 50°–53°. As a
   result of the second reflection, the colours of a secondary rainbow are
   inverted compared to the primary bow, with blue on the outside and red
   on the inside. The dark area of unlit sky lying between the primary and
   secondary bows is called Alexander's band, after Alexander of
   Aphrodisias who first described it.

   Some light reflects twice inside the raindrop before exiting to the
   viewer. When the incident light is very bright, this can be seen as a
   secondary rainbow, brightest at 50°–53°.
   Enlarge
   Some light reflects twice inside the raindrop before exiting to the
   viewer. When the incident light is very bright, this can be seen as a
   secondary rainbow, brightest at 50°–53°.
   A double rainbow features reversed colours in the outer (secondary)
   bow, with the dark Alexander's band between the bows.
   Enlarge
   A double rainbow features reversed colours in the outer (secondary)
   bow, with the dark Alexander's band between the bows.
   Rainbow in Moscow during the winter time is very rare
   Enlarge
   Rainbow in Moscow during the winter time is very rare

   A third, or triple, rainbow can be seen on rare occasions, and a few
   observers have reported seeing quadruple rainbows in which a dim
   outermost arc had a rippling and pulsating appearance. These rainbows
   would appear on the same side of the sky as the Sun, making them hard
   to spot.
   A contrast-enhanced photograph of a supernumerary rainbow, with
   additional green and purple arcs inside the primary bow.
   Enlarge
   A contrast-enhanced photograph of a supernumerary rainbow, with
   additional green and purple arcs inside the primary bow.
   Primary and secondary rainbows are visible, as well as a reflected
   primary and a faintly visible reflection primary.
   Enlarge
   Primary and secondary rainbows are visible, as well as a reflected
   primary and a faintly visible reflection primary.

   Occasionally, another beautiful and striking rainbow phenomenon can be
   observed, consisting of several faint rainbows on the inner side of the
   primary rainbow, and very rarely also outside the secondary rainbow.
   They are slightly detached and have pastel colour bands that do not fit
   the usual pattern. They are known as supernumerary rainbows, and it is
   not possible to explain their existence using classical geometric
   optics. The alternating faint rainbows are caused by interference
   between rays of light following slightly different paths with slightly
   varying lengths within the raindrops. Some rays are in phase,
   reinforcing each other through constructive interference, creating a
   bright band; others are out of phase by up to half a wavelength,
   cancelling each other out through destructive interference, and
   creating a gap. Given the different angles of refraction for rays of
   different colours, the patterns of interference are slightly different
   for rays of different colours, so each bright band is differentiated in
   colour, creating a miniature rainbow. Supernumerary rainbows are
   clearest when raindrops are small and of similar size. The very
   existence of supernumerary rainbows was historically a first indication
   of the wave nature of light, and the first explanation was provided by
   Thomas Young in 1804.

   Other rainbow variants are produced when sunlight reflects off a body
   of water. Where sunlight reflects off water before reaching the
   raindrops, it produces a reflection rainbow. These rainbows share the
   same endpoints as a normal rainbow but encompass a far greater arc when
   all of it is visible. Both primary and secondary reflection rainbows
   can be observed.

   A reflected rainbow, by contrast, is produced when light that has first
   been reflected inside raindrops then reflects off a body of water
   before reaching the observer. A reflected rainbow is not a mirror image
   of the primary bow, but is displaced from it to a degree dependent on
   the Sun's altitude. Both types can be seen in the image to the right.

History of the science of rainbows

   René Descartes' sketch of how primary and secondary rainbows are formed
   Enlarge
   René Descartes' sketch of how primary and secondary rainbows are formed
   Double rainbow over west Las Vegas, Nevada.
   Enlarge
   Double rainbow over west Las Vegas, Nevada.

   The Persian astronomer Qutb al-Din al-Shirazi (1236-1311), or perhaps
   his student Kamal al-din al-Farisi (1260-1320), is thought to have
   first given a fairly accurate explanation for the rainbow phenomenon .
   The work of Robert Grosseteste on light was continued by Roger Bacon,
   who wrote in his Opus Majus of 1268 about experiments with light
   shining through crystals and water droplets showing the colours of the
   rainbow. Theodoric of Freiberg is also known to have given 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 one reflection (at the back of the drop)
   before transmission into the eye of the observer" (quoted from David C,
   Lindberg, Roger Bacon's Theory of the Rainbow: Progress or Regress?,
   Isis, Vol. 57, no. 2, p. 235). He explained the secondary rainbow
   through a similar analysis involving two refractions and two
   reflections.

   Descartes, in 1637, further advanced this explanation. Knowing that the
   size of raindrops didn't appear to affect the observed rainbow, he
   experimented with passing rays of light through a large glass sphere
   filled with water. By measuring the angles that the rays emerged, he
   concluded that the primary bow was caused by a single internal
   reflection inside the raindrop and that a secondary bow could be caused
   by two internal reflections. He was able to back this up with a
   derivation of the law of refraction (subsequently, but independently
   of, Snell) and correctly calculated 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.

   Isaac Newton was the first to demonstrate that white light was composed
   of the light of all the colours of the rainbow, which a glass prism
   could separate into the full spectrum of colours, rejecting the theory
   that the colours were produced by a modification of white light. He
   also showed that red light gets refracted less than blue light, which
   led to the first scientific explanation of the major features of the
   rainbow. Newton's corpuscular theory of light was unable to explain
   supernumary 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.
   The end of a rainbow.
   Enlarge
   The end of a rainbow.

   Young's work was refined in the 1820s by George 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 published by Gustav Mie in
   1908. Advances in computational methods and optical theory continue to
   lead to a fuller understanding of rainbows. For example, Nussenzveig
   provides a modern overview^ .
   A double rainbow in a landscape oil painting by Peter Rubens.
   Enlarge
   A double rainbow in a landscape oil painting by Peter Rubens.

Rainbows in culture

Rainbows in religion and mythology

   The rainbow has a place in legend due to its beauty and the difficulty
   in explaining the phenomenon before the work of Descartes in the 17th
   century (although, as mentioned above, Theodoric of Freiburg had given
   a satisfactory explanation in the 13th century.)

   In Greek mythology, the rainbow was considered to be a path made by a
   messenger ( Iris) between Earth and Heaven. In Chinese mythology, the
   rainbow was a slit in the sky sealed by Goddess Nüwa using stones of
   five different colours. In Hindu mythology, the rainbow is called
   Indradhanush, meaning the bow of Indra, the God of lightning and
   thunder. In Norse Mythology, a rainbow called the Bifröst Bridge
   connects the realms of Ásgard and Midgard, homes of the gods and
   humans, respectively. The Irish leprechaun's secret hiding place for
   his crock of gold is usually said to be at the end of the rainbow
   (which is impossible to reach). In the Hebrew Bible, the rainbow is a
   symbol of the covenant between God and man, and God's promise to Noah
   that he would never again flood the entire Earth. The Rainbow has even
   become the symbol for a modern movement within Judaism called B'nei
   Noah. B'nei Noah are non-Jews who continue to follow in the ways of
   their ancestor Noah. The Noahide movement has its roots in Jewish
   Tradition, specifically the Talmud.

Rainbows in literature

   Virginia Woolf in To The Lighthouse highlights the transience of life
   and Man's mortality through Mrs Ramsey's thought,

   "it was all as ephemeral as a rainbow"

   An poem of William Wordsworth from 1802, " My Heart Leaps Up When I
   Behold The Rainbow", begins:

          My heart leaps up when I behold

                A rainbow in the sky:

          So was it when my life began;
          So is it now I am a man;
          So be it when I shall grow old,

                Or let me die!…

   "Harpe de Lumière" photo by Georges Noblet
   Enlarge
   "Harpe de Lumière" photo by Georges Noblet

   However, the Newtonian deconstruction of the rainbow is said to have
   provoked John Keats to lament in his poem " Lamia" in 1820:

          Do not all charms fly
          At the mere touch of cold philosophy?
          There was an awful rainbow once in heaven:
          We know her woof, her texture; she is given
          In the dull catalogue of common things.
          Philosophy will clip an Angel's wings,
          Conquer all mysteries by rule and line,
          Empty the haunted air, and gnomed mine –
          Unweave a rainbow

   In contrast to this is Richard Dawkins; talking about his book
   Unweaving the Rainbow: Science, Delusion and the Appetite for Wonder:

          "My title is from Keats, who believed that Newton had destroyed
          all the poetry of the rainbow by reducing it to the prismatic
          colours. Keats could hardly have been more wrong, and my aim is
          to guide all who are tempted by a similar view, towards the
          opposite conclusion. Science is, or ought to be, the inspiration
          for great poetry."

   The Rainbow is the title of a 1915 novel by British author D.H.
   Lawrence.

Rainbows in photography

   The rainbow is a favourite subject for photographers, to such an extent
   that photographs of rainbows seem to be more commonplace than rainbows
   themselves. Occasionally a rainbow photo will be surprisingly artistic
   and beautiful, as Georges Noblet's "Harpe de Lumière" shown to the
   right.

Rainbows in popular culture

   "Rainbow of Hearts" photo by Seng P. Merrill. In popular culture, the
   colours of the rainbow are sometimes permuted.
   Enlarge
   "Rainbow of Hearts" photo by Seng P. Merrill. In popular culture, the
   colours of the rainbow are sometimes permuted.
   The rainbow flag is an international gay and lesbian symbol.
   Enlarge
   The rainbow flag is an international gay and lesbian symbol.

   The rainbow has also been used in more contemporary settings, such as
   the song " Over the Rainbow" in the musical film The Wizard of Oz, the
   hit song " The Rainbow Connection" from The Muppet Movie, and in
   selling Lucky Charms by alluding heavily to leprechaun mythology.

   The Greenpeace ship, Rainbow Warrior, was named after a Cree Native
   American prophecy that stated "When the world is sick and dying, the
   people will rise up like Warriors of the Rainbow.…"

   Rainbow Coalition is a local social action group that was started in
   Chicago, Illinois by Jesse Jackson from which he launched his national
   political and social reform platform.

   Rainbow Gatherings are gatherings of hippies who come together on
   public lands with a stated mission to espouse the ideas of peace, love,
   freedom and community. In the sixties, artists like Peter Blake made
   use of Rainbow colours in iconic prints such as Babe Rainbow, and
   later, Bobbie Rainbow.

   In recent years, the rainbow flag has become a symbol of the gay and
   lesbian rights movement and gay pride, with the different colours
   symbolising diversity in the gay community. Historically, a rainbow
   flag was used in the German Peasants' War in the 16th century as a sign
   of a new era, of hope and of social change. Rainbow flags have also
   been used as a symbol of the Cooperative movement; as a symbol of
   peace, especially in Italy; to represent the Tawantin Suyu, or Inca
   territory, in Peru and Ecuador; by some Druze communities in the Middle
   east; and by the Jewish Autonomous Oblast.

Remembering the sequence of colours

   A traditional way of recalling each of the seven colours of the rainbow
   in sequence is by using the mnemonic "Richard Of York Gave Battle In
   Vain", where the first letter of each word corresponds to the first
   letter of each colour. The mnemonic probably alludes to the defeat and
   death of Richard, Duke of York at the Battle of Wakefield, a battle
   Richard had himself started, although it could also refer to the defeat
   of Richard III, the last Yorkist king, at the Battle of Bosworth Field.
   In York, an alternative version is taught that goes "Rowntrees Of York
   Gave Best In Value", referring to the confectionery Rowntrees. Another,
   less obvious, though more common way is the use of the meaningless name
   Roy G. Biv or ROYGBIV. This method is purely phonetic. Some humorous
   examples also exist, including "Ring Out Your Granny's Boots In
   Vinegar".

   All these mnemonics follow the tradition of including the colour indigo
   between blue and violet. Newton had inserted indigo probably to
   increase the number of colours to seven—a theologically preferred
   number. Some sources now omit indigo, partly due to the poor ability of
   humans to distinguish colours in the blue portion of the visual
   spectrum.^ Since rainbows are composed of a nearly continuous spectrum,
   different people, most notably across different cultures, identify
   different numbers of colours in rainbows.

   The colour sequence of a rainbow may also be recalled by electronics
   technicians through their knowledge in identifying the Electronic
   colour code.

   Retrieved from " http://en.wikipedia.org/wiki/Rainbow"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
