Why so we see rainbows ?

When sunlight passes through raindrops it is slightly bent. Sunlight is mixture of color – what we call a spectrum of colors. The raindrops bend some colors more than others, so they are separated out to make the colors of the rainbow.

Sunlight has to pass through the raindrops at a low angle for the colors to show as a semicircular bow. This is why you see rainbows most often after showers in the early morning or late evening , and not at midday. From an aircraft or a mountain top you can sometimes see a rainbow below you as a complete circle.