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 multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.
Why does the rainbow appear after the rain ?
The rainbow in the sky can be understood as an instance of the phenomenon of dispersion of light. While it is not strictly true, light from the Sun is almost white. This means that it is composed of all seven colors – violet, indigo,blue, green, yellow, orange and red. This is in increasing order of wavelength.
When white light from the sun enters a raindrop, some of it is reflected and rest is refracted. When it enters inside the raindrop, it may suffer total internal reflection at the back of the water droplet. It then refracts and exits. In this mechanism, the various wavelengths of light do not travel along the same path. This is because the path of the refracted ray depends on the refractive indices of both media. This is embodied in Snell’s law.
All components of white light would have travelled along the same path if refractive index were independent of wavelength. However, this is not the case, For light in the visible region, the refractive index decreases with increasing wavelength for the air-water media. Therefore, the white light splits up by the time it exits the water drop. It splits up into the component VIBGYOR colors, which form the rainbow.
Rainbows do not occur at fixed points – there are always many formed. However, from a fixed observer location, only one can be seen at a time. They are also usually full circles and not half arcs. It is the observer that just sees a portion of the arc of the refracting water drops, centered around the line joining the Sun and the observer’s eye.
Rainbows are always formed in the part of sky such that the observer’s back is to the Sun.