Rainbow From Wikipedia, the free encyclopedia For other uses, see Rainbow (disambiguation). Double rainbow and supernumerary rainbows on the inside of the primary arc. The shadow of the photographer's head on the bottom marks the centre of the rainbow circle (antisolar point). A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc. 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 in droplets 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 other rainbow, in both rainbows). This second rainbow is caused by light reflecting twice inside 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 for the region between a double rainbow to show any light reflected from water drops. It is impossible for an observer to maneuver 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" of a 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 is seen 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, is Newton'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. Rainbows may also form in mist, such as that of a waterfall Rainbow with a faint reflected rainbow in the lake Contents [hide] • 1 Visibility • 2 Spectrum • 3 Explanation • 4 Variations o 4.1 Double rainbow o 4.2 Supernumerary rainbow o 4.3 Reflected rainbow, reflection rainbow o 4.4 Monochrome rainbow o 4.5 Circumhorizontal arc o 4.6 Rainbows on Titan • 5 Scientific history • 6 Culture o 6.1 Religious Belief o 6.2 Art o 6.3 Literature o 6.4 Music o 6.5 Films o 6.6 Flags • 7 See also • 8 Notes • 9 References • 10 External links [edit] Visibility Rainbows may also form in the spray created by waves (called spray bows) Rainbow after sunlight bursts through after an intense shower in Maraetai, New Zealand. Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind at a low altitude angle. The most spectacular rainbow displays happen when half the sky is still dark with raining clouds and the observer is at a spot with clear sky in the direction of the sun. The result is a luminous rainbow that contrasts with the darkened background. The rainbow effect is also commonly seen near waterfalls or fountains. In addition, the effect can be artificially created by dispersing water droplets into the air during a sunny day. Rarely, a moonbow, lunar rainbow or nighttime rainbow, can be seen on strongly moonlit nights. As human visual perception for colour is poor in low light, moonbows are often perceived to be white.[1] It is difficult to photograph the complete semicircle of a rainbow in one frame, as this would require an angle of view of 84°. For a 35 mm camera, a lens with a focal length of 19 mm or less wide-angle lens would be required. Now that powerful software for stitching several images into a panorama is available, images of the entire arc and even secondary arcs can be created fairly easily from a series of overlapping frames. 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°. At good visibility conditions (for example, a dark cloud behind the rainbow), the second arc can be seen, with inverse order of colours. At the background of the blue sky, the second arc is barely visible. [edit] Spectrum A rainbow spans a continuous spectrum of colours—there are no "bands." The apparent discreteness is an artefact of the photopigments in the human eye and of the neural processing of our photoreceptor outputs in the brain. Because the peak response of human colour receptors varies from person to person, different individuals will see slightly different 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. The final colour in the rainbow is violet, not purple. Newton originally (1672) named only five primary colours: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven colours by analogy to the number of notes in a musical scale.[3] Red Orange Yellow Green Blue Indigo Violet A corresponding article in the Japanese Wikipedia may contain information and sources useful in building this article. (May 2011) Click [show] on the right for instructions.[show] [edit] Explanation This article may need to be rewritten entirely to comply with Wikipedia's quality standards. You can help. The discussion page may contain suggestions. (June 2011) The light is first refracted entering 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 40–42°. The angle is independent of the size of the drop, but does depend on its refractive index. Seawater has a higher refractive index than rain water, so the radius of a "rainbow" in sea spray is smaller than a true rainbow. This is visible to the naked eye by a misalignment of these bows.[4] The amount by which light is refracted depends upon its wavelength, and hence its colour. This effect is called dispersion. Blue light (shorter wavelength) is refracted at a greater angle than red light, but due to the reflection of light rays from the back of the droplet, the blue light emerges from the droplet at a smaller angle to the original incident white light ray than the red light. Due to this angle, blue is seen on the inside of the arc of the primary rainbow, and red on the outside. Contrary to popular belief, the light at the back of the raindrop does not undergo total internal reflection, and some light does emerge from the back. However, light coming out the back of the raindrop does not create a rainbow between the observer and the Sun because 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 rather than forming a rainbow.[5] Light rays enter a raindrop from one direction (typically a straight line from the Sun), reflect off White light separates into different colours the back of the raindrop, and fan out as they leave on entering the raindrop due to dispersion, the raindrop. The light leaving the rainbow is causing red light to be refracted less than spread over a wide angle, with a maximum intensity blue light. at 40.89–42°. A rainbow does not actually exist at a particular location in the sky. Its apparent position depends on the observer's location and the position of the Sun. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer's eye. This light is what constitutes the rainbow for 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 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 usually cannot be seen as there are not usually sufficient raindrops between the horizon (that is: eye height) and the ground, to contribute.