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== Fisika katumbiri ==
<!--Panémbong katumbiri mangrupakeun balukar tina [[dispersi]] cahya srangéngé nalika [[réfraksi|karéfraksikeun]] ku keclak cihujan (nu méh buleud). Cahyana mimitina diréfraksikeun nalika asup kana beungeut cihujan, [[reflection (physics)|reflects]] off the back of the drop, and is again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of [[angle]]s, with the most intense light at an angle of about 40°–42°, regardless of the size of the drop. Since the water of the raindrops is dispersive, the amount that the sunlight is bent depends upon the [[wavelength]] (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 [[spectra]] crosses itself, and thus 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. This is because the emitted spectra there do not have a maximum of intensity, as the other visible rainbows do. Thus the colours blend together and do not form any rainbow.
{|
| [[Image:rainbowrays.png|framed|Light rays enter 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 around 40°–42°]]
| [[Image:rainbow1.png|framed|White light separates into different colours (wavelengths) on entering the raindrop, as 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.]]
|}
A rainbow does not actually exist at a location in the sky, but 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. Its position is always in the opposite direction of the sun with respect to the observer, and the interior is actually a magnified image of the sun, which can be seen to be 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 (this means that if the sun is higher than 42° the rainbow is below the horizon and cannot be seen unless the observer is at the top of a mountain or a similar vantage point). Similarly it is difficult to photograph the complete arc of a rainbow, which would require an [[angle of view]] of 84°. For a [[135_film|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.
Sometimes, a second, dimmer rainbow is seen outside the primary bow, caused by a double reflection of the sunlight inside the raindrops, and appears at an angle of 50°–53°. Because of the extra reflection, the colours of the bow are inverted compared to the primary bow, with blue on the outside and red on the inside.
{|
|[[Image:rainbowrays2.png|frame|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°.]]
|[[Image:Double_Rainbow.jpg|frame|A double rainbow features reversed colours in the outer (secondary) bow.]]
|}
A triple rainbow is even more rarely seen. A few observers have reported seeing quadruple rainbows in which a dim outermost arc had a rippling and pulsating appearance.
Occasionally, another beautiful and striking rainbow phenomenon could 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 [http://www.jal.cc.il.us/~mikolajsawicki/rainbows.htm supernumerary rainbows], and their very existence was historically a first indication of the wave nature of light.
[[Image:Rainbows.jpg|left|thumb|Double rainbow.]]
The first accurate theoretical explanation of a rainbow came from [[Theodoric of Freiberg]] in 1307; he postulated that when sunlight falls on individual drops of moisture, the rays undergo two refractions (upon ingress and egress) and one reflection (at the back side 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. 236.).
[[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's law|Snell]]) and correctly calculated the angles for both bows. However, he was unable to explain the colours.
[[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 split into the full spectrum of colours. He also showed that red light gets refracted less than blue light which led to an essentially complete explanation of a rainbow's optical effect.
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== Katumbiri dina mitologi jeung ageman ==
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* [http://www.jal.cc.il.us/~mikolajsawicki/rainbows.htm Supernumerary and Multiple Rainbows]
[[Kategori:Optik]]
[[Kategori:Météorologi]]
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