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What is reason for getting rainbow as an arc ?
The colored arcs or the spectrum of white light from the sun in the form of bows seen immediately after rain by an observer with his back towards the sun is called a rainbow.An observer standing between the sun and the rainbow would see a 180 degree arc of colors which run through the visible spectrum from red to violet. In some cases, if conditions are right, double rainbows will form, with a pale rainbow in reversed colors above the primary rainbow, and in other instances, a fully circular rainbow has been observed, usually from inside an aircraft flying over the Earth.

The rainbow's appearance is caused by dispersion of sunlight as it is refracted by (approximately spherical) raindrops. The light is first refracted as it enters 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 about 40-42 degrees. The different colours (wavelengths) of light have different refractive indices which cause them to bend unequally. Thus, a rainbow is arc-shaped.


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