A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicolored circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun. Rainbows can be full circles. However, the observer normally sees only an arc formed by illuminated 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 refracted when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it. In a double rainbow, a second 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 twice on the inside of the droplet before leaving it. A rainbow is not located 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 see 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 seems "under" or "at the end of" a rainbow, the second observer will see a different rainbow, farther off, at the same angle as seen by the first observer. Rainbows span a continuous spectrum of colors. Any distinct bands perceived are an artifact of human color 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 colors seen by the human eye, the most commonly cited and remembered sequence is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet, remembered by the mnemonic, Richard Of York Gave Battle In Vain (ROYGBIV). Rainbows can be cause by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew. 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 usually seen in the western sky during the morning and in the eastern sky during the early evening. 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. During such good visibility conditions, the larger but fainter secondary rainbow is often visible. It appears about 10 degrees outside of the primary rainbow, with inverse order of colors. 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 color is poor in low light, moonbows are often perceived to be white. It is difficult to photograph the complete semicircle of a rainbow in one frame, as this would require an angle of view of 84 degrees. For a 35mm camera, a wide-angle lens with a focal length of 19mm or less would be required. Now that 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 above the earth such as in an airplane, 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 smaller, covering only 5-20 degrees. The sky inside a primary rainbow is brighter than the sky outside of the bow. This is because each raindrop is a sphere and it scatters light over an entire 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 most 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. A spectrum obtained using a glass prism and a point source is a continuum of wavelengths without bands. The number of colors that the human eye is able to distinguish is a spectrum is in the order of 100. Accordingly, the Munsell color system, a 20th-century system for numerically describing colors, based on equal steps for human visual perception, distinguishes 100 hues. The apparent discreteness of main colors is an artifact of human perception and the exact number of main colors is a somewhat arbitrary choice. Newton, who admitted his eyes were not very critical in distinguishing colors, originally divided the spectrum into five main colors: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven main colros by analogy to the number of notes in a musical scale. Newton chose to divide the visible spectrum into seven colors out of a belief derived from the beliefs of the ancient Greek sophists, who thought there was a connection between the colors, 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 seems merely deep blue." The color pattern of a rainbow is different from a spectrum, and the colors are less saturated. There is a spectral smearing in a rainbow owing to the fact that for any particular wavelength, there is a distribution of exit angle, rather than a single unvarying angle. In addition, a rainbow is a blurred version of the bow obtained from a point source, because the disk diameter of the sun cannot be neglected compared to the width of a rainbow. The number of color bands of a rainbow may therefore be different from the number of bands in a spectrum, especially if the droplets are particularly large or small. Therefore, the number of colors of a rainbow is variable. If, however, the word rainbow is used inaccurately to mean spectrum, it is the number of main colors in the spectrum. The question of whether everyone sees seven colors 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, more recent research suggests that the number of distinct colors observed and what these are called depend on the language that one uses with people whose language has fewer color words seeing fewer discrete color bands. 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 reflected 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 overall effect is that part of the incoming light is reflected back over the range of 0 degrees to 42 degrees, with the most intense light at 42 degrees. This angle is independent of the size of the drop, but does depend on its refractive index. Seawater has a higher refractive index that 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. The reason the returning light is most intense at about 42 degrees is that this is a turning point, light hitting the outermost ring of the drop gets returned at less than 42 degrees, as does the light hitting the drop nearer to its center. There is a circular band of light that all gets returned right around 42 degrees. If the sun were a laser emitting parallel, monochromatic rays, then the brightness of the bow would tend toward infinity at this angle, ignoring interference effects. But since the sun's brightness is finite and its rays are not parallel, the brightness does not go to infinity. Furthermore, the amount by which light is refracted depends upon its wavelength, and hence its color. This effect is called dispersion. Blue light 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. The result of this is not only to give different colors to different parts of the rainbow, but also to diminish the brightness. 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 of 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 colors blend together rather than forming a rainbow.