Why do rainbows come only after rainfall?

Refraction is when light is bent, as is is passing from one medium to another. So with a rainbow we have light from the sun which is basically being seen through water, millions of tiny droplets usually. The arch of the rainbow is because it is bening bent over a series of angles.
There no medium for refraction before rain thus it apperas only after rains.
The reason that we see red, orange, yellow, blue, indigo and violet is because it is a near continuous spectrum of light in the sky.

Frequently rainbows are actually double rainbows, although the second rainbow may not always be ver visible. The second is always much fainter than the first and the colours bend round in the opposite direction, namely violet to red.

Various cultures have beliefs or myths about rainbows, from the 'pot of gold' at the end of the rainbow, to more ancient beliefs. For example in Norse Mythology a rainbow connects the homes of the Gods to the land of the humans!

 

Hope this helps,

Topper Learning