You get rainbows when the sun's rays are reflected through raindrops, which are a sort of 'prism' of nature. In the top picture, the sunlight is 'bouncing' off the inside of the raindrop just once, which gives a bright rainbow. In the bottom picture the light is 'bouncing' off twice, which gives a second rainbow above the first, but it is much paler and the colours run the other way round. At one time or another we have all seen a rainbow.
Rainbows happen when the sun is shining while it is raining or just after a storm.
It appears in the part of the sky directly opposite to the sun. So you must stand with your back to the sun to see it. The lower the sun is to the horizon, the higher the arch of the rainbow will be. The light that we see coming from the sun or an electric light bulb appears to be clear and white. But it is really made up of bands or waves of colours.