we know when light from the sun (white light) passes through a glass prism the light is broken into colours, called the spectrum, from red to violet (Newton showed this). This happens as light of different colour passes at different speed through the prism and this then gives the effect of seeing different colours, that is the prism breaks up the light into the rainbow colours. This is called refraction
A rainbow is similar. the light first acts like it is going through a prism (each water droplet acts in this way) so it is broken up into the colours red organge yellow green blue indigo violet. Then the light is reflected back to us off the back of the water drop, but not as the original white light. Instead it is reflected back split into these colours. So you then see the white light split into its colours.
Sun gives the light to be split and rain gives the prism or water droplets.
Same effect as you see with oil in a puddle on a sunny day.
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