VALENCIA. Although raindrops are usually represented in the shape of a teardrop, actually, they are almost circular. When a beam of light enters a raindrop, its light separates into colours (as it happens when light goes through glass, for instance) and changes its direction. When this light gets to the other side of the droplet and tries to get out of it, a small fraction of it does not reach the other side and it is reflected backwards, almost coming out of the raindrop from where it had previously entered.
That is, raindrops reflect backwards a small percentage of the light that comes through them, separating it into colours. But saying backwards is not exactly accurate. Since raindrops are curved, when light is reflected and refracted once and again in this curvy area, light is reflected at an angle of 138º with respect to incidental light. This is the angle that explains the rainbow.
Suppose the following situation. The Sun is behind us, near the horizon, and, in the distance, in front of us it is raining, and the Sun throws its light on these drops. All of them are reflecting backwards part of this sunlight, separated into colours. However, we will not be able to perceive this light coming from every raindrop, only from some of them, from those that, regarding my position, make exactly an angle of 138º with respect to sunlight, for only in this case will the reflected light reach our eyes.
Every point in a curtain of rain that, regarding my position, make an angle of 138º with respect to the sunlight make up a hoop or a ring. Therefore, we should actually call it Rainhoop, but the lower part of this hoop disappears in the ground (where there are no raindrops): that is why we see a bow. Sometimes, it is possible to see the whole hoop. For instance, if we are at the edge of a waterfall and the Sun is behind us and the foam the waterfall produces is in front of us. We can also make it happen: if we take a garden hose with a mouthpiece that mists water, and we start it up in front of us with the Sun behind us, we will see the whole hoop.
Full text available at Mètode's website.
Fernando Ballesteros. Astronomical Observatory of the University of Valencia