Sometimes an arc of spectral colours appears in the sky after a rainfall. This is called rainbow. The viewers can see the rainbow only when the sun is at the back of the observer and the sun rays fall obliquely which is possible only in the morning or evening. Rainbow is the most beautiful and colourful natural phenomena which had been fascinating man from ancient times.
In reality rainbow is a solar spectrum produced due to the dispersion of light. After the rain, innumerable droplets of water remain suspended high up in the air. When sun rays fall on the droplets, each droplet behaves like a prism and light rays enterning the drops suffer refraction and total internal reflection. Since the rays are of white light so they split up into seven colours.
The rainbow described above has brightest colours and is easily visible. This is called “Primary Rainbow” in this case light suffers only one internal reflection in rain drops (fig. 14. 12-a). The primary rainbow has red colour on its outer and violet colour on its inner edge. The order of the colours present in the primary rainbow are red, orange, yellow, green, blue, indigo and violet.
Sometimes a secondary rainbow beyond the outer edge of the primary rainbow is seen. Secondary rainbow is fainter than the primary rainbow and is seldom visible. This is produced when the rays from the sun suffer double internal reflections. Double internal reflections in the water droplets cause the colour bands of the secondary rainbow to be in reverse order than is the primary rainbow, i.e., the outer edge of the secondary rainbow is violet and the inner edge is red. Thus in the secondary rainbow. The order of the colours is violet, indigo, blue, green, yellow, orange and red.
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