Rainbow , series of concentric coloured arcs that may be seen when light from a distant source—most commonly the Sun —falls upon a collection of water drops—as in rain, spray, or fog. The rainbow is observed in the direction opposite to the Sun. The coloured rays of the rainbow are caused by the refraction and internal reflection of light rays that enter the raindrop , each colour being bent through a slightly different angle. Hence, the composite colours of the incident light will be separated upon emerging from the drop. The most brilliant and most common rainbow is the so-called primary bow, which results from light that emerges from the drop after one internal reflection. Although light rays may exit the drop in more than one direction, a high density of the rays emerge at a minimum angle of deviation from the direction of the incoming rays. Occasionally, a secondary bow may be observed, which is considerably less intense than the primary bow and has its colour sequence reversed. This bow results from light that has undergone two internal reflections within the water drop. Higher-order rainbows, resulting from three or more internal reflections, are exceedingly weak and hence are rarely observed.
Where is the sun when you see a rainbow?
What is a rainbow?
A rainbow is a multicolored arc in the sky which appears when sunlight hits water droplets. How does it get its colors? Why is it curved? And what is at the end of the rainbow? Rainbows always appear in the sky opposite to the Sun. So, if you have your back to the Sun, the rainbow will arch across the sky in front of you. A rainbow is an optical phenomenon which involves three processes: reflection, dispersion, and refraction. Water droplets can act like little mirrors.
If you had no idea at all about what a rainbow is or what causes it, you might actually believe some of the legends that different ancient cultures have created to explain it. A rainbow requires water droplets to be floating in the air. The Sun must be behind you and the clouds cleared away from the Sun for the rainbow to appear. A full rainbow is actually a complete circle, but from the ground we see only part of it. From an airplane, in the right conditions, one can see an entire circular rainbow. The sunlight shines on a water droplet. As the light passes into the droplet, the light bends, or refracts, a little, because light travels slower in water than in air because water is denser. Then the light bounces off the back of the water droplet and goes back the way it came, bending again as it speeds up when it exits the water droplet.
A rainbow is a meteorological phenomenon that is caused by reflection , refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun. Rainbows can be full circles. However, the observer normally sees only an arc formed by illuminated droplets above the ground,  and centered on a line from the sun to the observer's eye.