what causes a rainbow?
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Sunlight passing through water. As the light passes through the rain, it is divided into the individual colors of thr light spectrum, so you can see all the different colors of the sunlight's light spectrum.
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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.\nIn 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 perceived to be white.\nThe 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°, 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, 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.\n 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 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 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.\nFrom 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.
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Like this?\nhttp://www.webelements.com/webelements/elements/media/pics/rainbow.jpg\n\nTo quote the web site,\n"The picture above shows the result of adding different metal salts to a burning reaction mixture of potassium chlorate and sucrose. The red colour originates from strontium sulphate. The orange/yellow colour originates from sodium chloride. The green colour originates from barium chlorate. The blue colour originates from copper (I) chloride. The lilac colour that should be evident from the potassium chlorate is washed out by the other colours, all of which are more intense."\n\nBasically, certain salts will color the flames various colors due to how the electrons are excited (more complicated than that, look into spectroscopy).
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An inverse rainbow is when the colors are reversed so that the red band of color is on the inside of the rainbow and the blue band is on the outside.  Moonbows, or, halos, are inversed. I wasn't aware that rainbows were different here and there. If they are, my guess is the explanation would be found in optics and, maybe bioelectric fields stuff? Here's a GREAT website for future reference and to put in your Favorites file.  Just click on any "circle" of science and follow the algorithm. Handy little tool. Can't wait to come back and read if someone has an explanation.\nhttp://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
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The arc of a rainbow has nothing to do with the curvature of the earth.\n\nA rainbow is formed because light from the sun is double refracted back toward you. When you see a rainbow the sun will be behind you, and the centre of the rainbow will be opposite the position of the sun.\n\nOf course, the light is scattered back equally in every direction you look, so a rainbow is actually a full circle with its centre opposite the sun.\n\nHowever, unless you are in a plane looking down (when you do see a full circle - i've seen it) the point opposite the sun will be below the horizon, so part of the circle will be cut off by the horizon and you will see an arc.
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It's called a lunar halo.  It is caused by refraction of light by water droplets in the atmosphere, in much the same way that a rainbow is caused, and there is a solar halo, as well, at times.  if you see it again, place your closed fist at arm's length at one side of the halo and see how many fists it takes to get across to the other side.  It should be two, plus about two fingers worth, 22.5 degrees diameter (same for the solar halo).  At times the solar halo is broken into two very bright spots 11 degrees or so either side of the sun:  these are popularly called "sun dogs".  If thin ice crystal clouds are the cause there is usually no coloration associated with the halo.
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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.
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The sunlight spectrum is made of all colors of the rainbow, and seen as light because all the colors are present at the same time. \n\nWater magnifies the visibility of this light and we are able to see the colors of light distinct and separated into the actual colors of sunlight.
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1. Red, orange, yellow, green, blue, indigo, and violet. Actually, the rainbow is a whole continuum of colors from red to violet and even beyond the colors that the eye can see.\n2. you can see a rainbow anywhere there is water around and you have the sun to your back.\n3.Sir Isaac Newton in his early physics experiments decided the colors of the rainbow
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A rainbow is created when white light from the sun enters droplets of water in the sky and is refracted. The light is refracted when entering spherical droplets, then it is reflected inside them and refracted again upon leaving. This bends the light, as you would see in a lens.  The red end of the spectrum bends less than the blue, which is why they separate when they come out of the drop.\nWikipedia has a page about rainbows, with some diagrams which may help you understand.\nhttp://en.wikipedia.org/wiki/Rainbow
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a rainbow? stuffed if i know....lol
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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.\n\nEven 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.\n\nScientific explanation\n\nThe 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.\n\nIn 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.\nRainbows may also appear in the spray of a water fountain\nEnlarge\nRainbows may also appear in the spray of a water fountain\n\nThe 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 [1].\n\nSince 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.\n\nA 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 obser
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the other answers are somewhat correct but unnecessarily long....\n\n1. the rainbow is actually a circle that is partially visible as an arch due     \n    horizon limitations.\n\n2.  it is a circle because the light source ( the sun ) is a circle.
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A Corona.\n\nThe moon can produce interesting optical effects when conditions are right. The most common of which are moon rings, moon bows, which are similar to rainbows, moon dogs and moon pillars. A rainbow is produced when sunlight is refracted through water droplets - A similar effect is produced when moon light refracts through ice crystals\n\nThe ring around the Moon is caused by the refraction of Moonlight (which of course is reflected sunlight) from ice crystals in the upper atmosphere. The shape of the ice crystals results in a focusing of the light into a ring. Since the ice crystals typically have the same shape, namely a hexagonal shape, the Moon ring is almost always the same size.
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The main colors of a rainbow are red, orange, yellow, green, blue, and purple, but there isnt usually that much purple. By the way, that list is from outside to inside.
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between 10am -2pm the sun is high in the sky. with a high hummity it is posible to see a rainbow all the way  around the sun
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You know how you see a rainbow when its really humid, or when it rains and there is sun? Thats what you saw, you saw a rainbow, but not a half rainbow, but rather a whole rainbow around the sun. A ring rainbow, this is due because of the high humidity that is in the air, its the same as a rainbow, just around the sun.