rainbow

A rainbow spans a continuous spectrum of colours; the discrete bands are an artefact of human colour vision

Rain is liquid precipitation. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface. Rain is the primary source of fresh water for most areas of the world, providing suitable conditions for diverse ecosystems, as well as water for hydroelectric power plants and crop irrigation.

Why do rainbows come after it has rained?

A rainbow spans a continuous spectrum of colours. Traditionally, however, the sequence is quantised. The most commonly cited and remembered sequence, in English, is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet. "Roy G. Biv" and "Richard Of York Gave/Gained Battle In Vain" are popular mnemonics.