Why semicircular rainbow? People have long zadavalis this issue. In some myths African rainbow is a snake that covers the Ground ring. But now we know that a rainbow is an optical phenomenon - the result prelamanja rays of light in water droplets in the rain. But why do we see a rainbow in the shape of an arc, and not, for example, in the form of vertical color stripes?
The rainbow is determined by the form of water droplets, which refracted the sunlight. And drops of water, more or less spherical (round). Passing through the drop and refracting it, a beam of white sunlight is converted into a series of coloured funnels, inserted one into the other, facing the observer. The outer red funnel, inserted in orange, yellow, followed by green and so on, ending with the inner purple. Thus, each drop forms a rainbow.
Of course, a rainbow from one drop weak, and in nature it cannot be seen separately, as drops in the veil of the rain a lot. The rainbow that we see in the sky, formed by myriads of drops. Each drop creates a series of nested one in another color funnels (or cones). But from individual drops in the rainbow gets only one color beam. The observer's eye is a common point where it is crossed colored rays from many drops. For example, all red rays coming from different drops, but under the same angle and caught in the eye of the observer, form a red arc of the rainbow. They arc all orange and other color rays. So rainbow round.
Two people standing close to each other, see each of your rainbow! Because in every moment of the rainbow formed by the refraction of sunlight in the new drops. Rain drops are falling. Place the falling drops is different and manages to send their colored rays in the rainbow, followed by the next and so on.
View rainbow - the width of the arcs, the presence, location, and brightness of individual colors, the position of the additional arcs is very much dependent on the size of raindrops. The larger drops of rain, the narrower and brighter it turns out the rainbow. Characteristic of large drops is the presence of a deep red color in the main rainbow. Numerous additional arcs also have the bright tone and directly, without gaps, are adjoined to the rainbows. Than smaller drops, the rainbow becomes wider and faded with orange or yellow edge. Additional arc further separated and apart from each other and from the main rainbows. Thus, in view of the rainbow can roughly estimate the size of the drops of rain that formed the rainbow.
The effect depends on the shape of the drops. When fall is in the air big drops plushevaya, lose their sphericity. The stronger the flattening of the droplets, the smaller the radius formed by them of the rainbow.
We used to see a rainbow arc. Actually this arc is only a part of the colored circle. Entirely, this natural phenomenon can be observed only at high altitude, for example, from the aircraft.
There is such a group of optical phenomena, which is called halo. They are caused by refraction of light rays in a tiny ice crystals in Cirrus clouds and fogs. Often the halo formed around the Sun or moon. Here is an example of this phenomenon - spherical rainbow around the Sun: