- published: 31 Mar 2016
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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 "primary rainbow", the arc shows red on the outer part, and violet on the inner side. This rainbow is caused by light being refracted while 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 colours reversed, red facing toward the other one, in both rainbows. This second rainbow is caused by light reflecting twice inside water droplets.
The rainbow is not located at a specific distance, but comes from any water droplets viewed from a certain angle relative to the Sun's rays. Thus, a rainbow is not a physical object, and cannot be physically approached. Indeed, it is impossible for an observer to manoeuvre to see any rainbow from water droplets at any angle other than the customary one of 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.
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.
The Earth's outer solid layer, the lithosphere, is made of rock. Three majors groups of rocks are defined: igneous, sedimentary, and metamorphic. The scientific study of rocks is called petrology, which is an essential component of geology.
Rocks are generally classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them. These indicators separate rocks into three types: igneous, sedimentary, and metamorphic. They are further classified according to particle size. The transformation of one rock type to another is described by the geological model called the rock cycle.
Igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic rock and volcanic. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth's crust (example granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta (examples pumice and basalt).