The rainbow is one of the most well-known optical effects related to weather. Who, as a child, was not excited to see a rainbow in the sky and marvel at its beauty? The colors of the rainbow can be so pure and brilliant that no photo does justice in imaging this phenomenon.

Contrary to what many people think, even scientists, the rainbow remains not very well understood, even though books and many articles have been written about it. We know how it is formed, generally; but the literature contains numerous observations (even photos) of rainbow phenomena that can't be explained well or at all.

Following are a number of questions that people around me occasionally ask about the rainbow, with explanations.

1. What causes the rainbow?

   The rainbow is caused by light from the sun (or moon) interacting with raindrops falling from the air. These raindrops are mostly round. A ray of light entering such a droplet is bent (refracted) and decomposed into all possible colors (wavelengths) which the white light consists of. The ray then reflects internally in the droplet, and emerges roughly into the direction it came from when it entered the drop. Since the rays of the different colors all exit at slightly different directions, you see a color band in the sky.

2. Why is the rainbow colored?

   The colors are produced by a phenomenon called dispersion. Dispersion causes white light, which consists of all possible colors (better: wavelengths or frequencies) to break up into all components, when the light travels from one medium (e.g. air) to another (e.g. water). The different colors of light have all slightly different directions, so if you look in a certain direction in the sky from which mostly some particular colored light comes, you see that color.

   The rainbow forms in water droplets, which do not need to come from a rainstorm. You can sometimes see rainbows in lawn sprinklers, fountains and in the spray water of waterfalls (as in the picture) as well.

3. Why is the rainbow round?

   The rainbow appears round for two reasons. First, because the raindrops in which it is visible, are mostly round. Falling raindrops do not have the typical teardrop shape, but are more or less round. Second, the light from the sun is being refracted ("bent") by the droplets over a certain angle. This angle is so big that the rainbow appears opposite the sun. Since this angle is constant for all orientations of raindrops with respect to you and the sun, the rainbow is a circular bow, centered on the point opposite the sun (where the shadow of your head is). It is usually only seen above the horizon, since you are usually on the ground and there is no rain falling below you. In some circumstances, when the rainfall is very dense, you can see the rainbow in front of the ground, below the horizon.

4. There was sunshine and rain, but no rainbow??

   The rainbow only occurs when there is sun- or moonshine, and rainfall in the direction of the rainbow (the rainbow is a bow with radius 42 degrees, centered around the point opposite the sun, where the shadow of your head is). If the sun is not illuminating the raindrops, there will not be a rainbow. Also, when the sun is too high in the sky (higher than about 42 degrees altitude), the rainbow is below the horizon and you will probably not see it.

5. Where is the end of the rainbow?

   The rainbow has no end in the sense that it would be localized. The rainbow will move with you, and hence is never connected to the landscape. The landscape has nothing to do with the position of the bow. Moreover, the bow is three-dimensional. If rain was falling everywhere, the rainbow would start in the drops right in front of your face and extend to infinite distance, as a cone with its apex at your eye. In other words, the intersection of one of the bases of the rainbow with the ground (where the gold box would be) is not a point but a line, which can stretch for miles (as far as the rain is being illuminated by the sunlight).

   The secondary rainbow lies just outside the primary rainbow and has the order of the colors reversed.

6. Why does the area inside the rainbow look brighter than outside?

   The light that causes the rainbow emerges out of water droplets at every possible angle up to the radius of the rainbow. So, the light that interacts with the droplets and which reaches your eyes, forms a disk of light, with the colored rainbow at its perimeter. Outside the rainbow, there is no light exiting raindrops in the way that produces the main rainbow. Hence this area appears darker than the area within the bow. The dark area outside the rainbow is called Alexander's dark band.

7. What is a red rainbow? What causes it?

   White light consists of all wavelengths of light (the whole continuous range) that we can see with our eyes. The rainbow forms in water droplets that decompose this white light into the range of wavelengths (colors). If the sun is low in the sky, the sun is reddened due to atmospheric adsorption (by dust) and by Rayleigh scattering (responsible for the blue sky). That means that only the red part of the color spectrum causes the rainbow, and hence the bow appears reddened.

8. I saw two rainbows??

   There are infinitely many rainbows, with different radii. You probably saw the secondary rainbow, which lies just outside the first bow. The secondary bow has a radius of about 51 degrees. The tertiary and higher-order bows are never seen, since they are too faint and some are in a bright part of the sky (near the sun). You will note that the colors of the secondary bow are reversed from the primary bow.

9. I saw a rainbow at night that was white??

   This bow was probably caused by the moon. The moon is equally capable of producing rainbows, since all it takes to make a rainbow is a mass of water droplets and a light source. However, the lunar rainbow is usually so faint that you don't see colors, since we humans only see colors if the light is bright enough.

10. I saw a bright rainbow right overhead, but there was no rain falling??

    What you saw was not the rainbow, but a halo, most likely the circumzenith arc. Halos form in ice crystals, which are equally capable of producing colored arcs like the rainbow. These form in different parts of the sky, however.

11. I saw a rainbow with several violet and green color bands inside??

    These color bands are called the supernumerary bows and are a consequence of the wave-nature of light. The bands are caused by a phenomenon called light interference. Interference happens whenever waves of some sort in a medium interact with eachother. At some places, the waves amplify eachother, and at other places they extinguish eachother. Here, the waves are light waves, and the medium is air. The supernumeraries form when the raindrops are much of the same size.

12. I saw a rainbow that was straight (not round)??

    You probably saw a small section of the rainbow, maybe with the sun low in the sky. If the rain-producing storm was in the distance, you'd have seen a short vertical pillar, which is just the base of the rainbow where it intersects Earth.

13. What are the spokes that can sometimes be seen in the rainbow?

    These are solar rays, seen from behind (solar rays are usually seen diverging from the sun). Since these rays are opposite the sun (where the rainbow is), they are called antisolar or anticrepuscular rays. The region inside the rainbow is a region of high scattering of light, so these anticrepuscular rays are not uncommon to see in rainbows.

14. I saw two rainbows crossing eachother??

    You may have seen the rare reflection rainbow. This rainbow is a horizon-mirror image of the main rainbow and is produced when the sun reflects in a very flat water surface. The reflection makes two light sources, the sun and its mirror image below the horizon, and a rainbow is produced by each source.