|MadSci Network: Physics|
Remember that the water droplets act as prisms; they literally focus the various wavelengths of light. Different wavelengths of light have different energies, and are diffracted (bent) at different and distinct angles as they pass through. The different wavelengths give rise to the different colors we see. Short wavelengths are the violet end of the spectrum; long wavelengths are the red end. Now, why do they fall into such neat bands in the sky? It is because the light is all coming from the same direction -- the sun. The rays strike spherical water droplets at the same angle, and each droplet focuses the diffracted light in the same direction. You only see the visible light bands appear when the light is focused in the right direction.
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