A rainbow is a multi-coloured arc that forms in the sky. Rainbows happen when sunlight and rain combine in a very specific way. 
Rainbows are created by both reflection and refraction (bending) of light in water droplets in the atmosphere, which results in a spectrum of light appearing.
A rainbow is not an object, it cannot be approached or physically touched.
A rainbow is in fact a full circle of light. The bottom half of the rainbow is hidden from most observers because it is below the horizon. Occasionally passengers in a low-flying aircraft see rainbows that form complete circles.
No two people see the same rainbow. In fact even our individual eyes see slightly different rainbows. 
Rainbows can be seen not just in rain but also mist, spray, fog, and dew, whenever there are water drops in the air and light shining from behind at the right angle.
Sir Isaac Newton identified the 7 colours of the visible spectrum that together make up white light. There are seven colours in the rainbow: red, orange, yellow, green, blue, indigo and violet. (the acronym ROY G BIV is a good way to remember these colours and their order).
A “double rainbow” is a rare phenomenon where a second, much fainter arc can be seen outside of the primary arc. Interestingly, the colours of the main rainbow are ROYGBIV and the second VIBGYOR. The colours in the second rainbow are in the reverse order. The rays of light have undergone one more reflection and are reversed in the second rainbow.