Rainbow
A rainbow is a meteorological phenomenon that is caused by reflection, and dispersionof light in water droplets resulting in a spectrum of light appear in the sky. It takes the form of a many coloured circular arc. Rainbows caused by sunlight always appear in the section of sky opposite the sun.
Rainbows can be full circles. However, the observer normally sees only an arc formed by remove droplets above the ground, and centered on a line from the sun to the observer's eye.
In a primary rainbow, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being reflect when entering a droplet of water, then reflected inside on the back of the droplet and refracted again and again when leaving it.
In a double rainbow, a 2nd arc is seen outside the primary arc, and has the order of its colors reversed, with red on the inner side of the arc. This is caused by the light being reflected double on the inside of the droplet before leaving it.
Contents
- 1Overview
- 2Visibility
- 3Number of colours in spectrum or rainbow
- 4Explanation
- 5Variations
- 5.1Multiple rainbows
- 5.2Twinned rainbow
- 5.3Full-circle rainbow
- 5.4Supernumerary rainbows
- 5.5Reflected rainbow, reflection rainbow
- 5.6Monochrome rainbow
- 5.7Higher-order rainbows
- 5.8Rainbows under moonlight
- 5.9Fogbow
- 5.10Circumhorizontal and circumzenithal arcs
- 5.11Rainbows on Titan
- 5.12Rainbows with different materials
Overview
A rainbow is not place at a specific distance from the observer, but comes from an optical illusion caused by any water droplets viewed from a certain angle relative to a light source. Thus, a rainbow is not an object and cannot be physically approached. Indeed, it is impossible for an observer to look a rainbow from water droplets at any angle other than the customary one of 42 degrees from the direction opposite the light source. Even if an observer sees another observer who look "under" or "at the end of" a rainbow, the 2nd observer will see a other rainbow—farther off—at the same angle as look by the first observer.
Rainbows span a continuous spectrum of colours. Any distinct bands perceived 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 towards the other side. For colours seen by the human eye, the most commonly cited and remembered sequence is Newton's sevenfold red, orange, yellow, green, blue, and violet, remembered by the mnemonic,
Rainbows can be caused by most type of airborne water. These include not only rain, but also mist, spray, and airborne dew.
Visibility
Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind the observer at a low altitude angle. Because of this, rainbows are seen in the western sky during the morning and in the eastern sky during the early evening. The most spectacular rainbow displays 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. During such good visibility conditions, the larger but fainter secondary rainbow is often looking. It appears about 10° outside of the primary rainbow, with inverse order of colours.
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.
It is strict to photograph the all semicircle of a rainbow in one frame, as this would require an angle of view of 84°. For a thirty five mm camera, a vast-angle lens with a focal length of 19 mm or less would be neend. Now that software for stitching several images into a panorama is available, images of the all arc and even 2ndary arcs can be make fairly easily from a series of overlapping frames.
From above the earth such as in an aeroplane, it is sometimes possible to see a rainbow as a full circle. This phenomenon can be confused with the glory phenomenon, but a glory is usually much small, covering only 5–20°.
The sky inside a primary rainbow is shining than the sky outside of the bow. This is because each raindrop is a sphere and it scatters light over an all circular disc in the sky. The radius of the disc depends on the wavelength of light, with red light being scattered over a larger angle than blue light. Over many of the disc, scattered light at all wavelengths overlaps, resulting in white light which brightens the sky. At the edge, the wavelength dependence of the scattering gives rise to the rainbow.
Light of primary rainbow arc is 96% polarised tangential to the arch.[6] Light of 2ond arc is 90% polarised.
Number of colours in spectrum or rainbow
A spectrum get using a glass prism and a point source is a continuum of wavelengths without bands. The number of colours that the human eye is able to distinguish in a spectrum is in the order of 100. Accordingly, the Munsell colour system distinguishes hundred hues. The apparent discreteness of main colours is an artefact of human perception and the exact number of main colours is a somewhat arbitrary choice.
| Red | Orange | Yellow | Green | Blue | Indigo | Violet |
Newton, who admitted his eyes were not very critical in different colours, originally (1672) divided the spectrum into 5 main colours: red, yellow, green, blue and violet. Later he included orange and indigo, giving 7 main colours by analogy to the number of notes in a musical scale. Newton chose to divide the visible spectrum into seven colours out of a belief derived from the beliefs of the ancient Greek sophists, who thought there was a connection between the colours, the musical notes, the known objects in the Solar System, and the days of the week.
According to Isaac Asimov, "It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes it look merely deep blue.
The colour pattern of a rainbow is different from a spectrum, and the colours are less saturated. There is spectral smearing in a rainbow owing to the fact that for any particular wavelength, there is a distribution of exit angles, rather than a single unvarying angle. a rainbow is a blurred version of the bow obtained from a point source, because the disk diameter of the sun (0.5°) cannot be ignore compared to the width of a rainbow (2°). The number of colour bands of a rainbow may therefore be different from the number of bands in a spectrum, especially if the droplets are particular large or small. Therefore, the number of colours of a rainbow is variable. If, however, the word rainbow is used inaccurately to mean spectrum, it is the number of main colours in the spectrum.
The question of whether everyone sees 7 colours in a rainbow is related to the idea of Linguistic relativity. Suggestions have been made that there is universality in the way that a rainbow is perceived.However, most recent research suggests that the number of distinct colours observed and what these are called depend on the language that one uses with people whose language has fewer colour words seeing fewer discrete colour bands.
Explanation
When sunlight encounters a raindrop, part of the light is reflected and the rest enters the raindrop. The light is refracted at the surface of the raindrop. When this light hits the back of the raindrop, some of it is reflect off the back. When the internally reflected light reaches the surface again, once more some is internally reflected and some is refracted as it exits the drop. (The light that reflects off the drop, exits from the back, or continues to bounce around inside the drop after the second encounter with the surface, is not relevant to the formation of the primary rainbow.) The all to all effect is that part of the incoming light is reflected back over the range of 0° to 42°, with the more intense light at 42°. This 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 small than a true rainbow. This is looking to the naked eye by a misalignment of these bows.
The reason the returning light is most intense at about 42° is that this is a turning point – light hitting the outermost ring of the drop gets returned at less than 42°, as does the light hitting the drop nearer to its centre. There is a circular band of light that all gets returned right around 42°. If the sun were a laser emitting parallel, monochromatic rays, then the luminance (brightness) of the bow would tend toward infinity at this angle (ignoring interference effects). .) But since the sun's luminance is finite and its rays are not all parallel (it covers about half a degree of the sky) the luminance does not go to infinity. Furthermore, the amount by which light is refracted depends upon its wavelength, and its colour. This effect is called dispersion. Blue light (shorter wavelength) is refracted at a great 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 look on the inside of the arc of the primary rainbow, and red on the outside. The result of this is not only to give different colours to different parts of the rainbow, but also to diminish the shining. (A "rainbow" formed by droplets of a liquid with no dispersion would be white, but shining than a normal rainbow.)
The light at the back of the raindrop does not undergo total internal reflection, and some light does emerge from the back. However, light come out the back of the raindrop does not make 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 look rainbows do, and thus the colours blend together rather than forming a rainbow.
A rainbow does not exist at 1 particular location. Many rainbows include; however, only one can be seen depending on the particular observer's viewpoint as droplets of light illuminated by 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.
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