A rainbow is caused by reflection of light in water droplets in the atmosphere, resulting in a spectrum of light appearing in the sky in the form of a multicolored arc. Now this much we probably all know, but here are some interesting facts about this beautiful natural phenomenon which you may not know:
Position/Angle: A rainbow is not an object located at a specific distance, thus it cannot be physically approached. It is impossible for an observer to move himself into a position to see the rainbow from any other angle other than the customary 42 degrees from the direction opposite the sun. If you see someone closer to a rainbow, as in you see them under the rainbow you are seeing, they will be seeing a different rainbow further away maintaining the same angle of viewing.
Center: A rainbows center is at the shadow of the observers head (the antisolar point). The bow then forms a circle with an angle of 42 degrees to the line between the observers head and the shadow of his head. If the sun itself is higher than 42 degrees then the rainbow usually cannot be seen as it falls behind the horizon. Exceptions to this can occur when the observer is high up compared to the horizon (in a plane, or up a mountain).
Colors: A rainbow spans a complete, continuous spectrum of colors. Any distinct bands of color we see are from our own color vision capabilities. When viewed in black & white a rainbow doesn’t have any bands and it is a continuous gradient.
Double Rainbow: A double rainbow has a second arc seen outside of the main arc with the order of its colors reversed. The second rainbow is caused by light reflecting twice inside the water droplets. For this second rainbow to be seen decently a dark background is required (a dark cloud behind the rainbow) otherwise the second rainbow is quite faint and may not be visible.
Best Viewing: The best view of a rainbow is when half the sky is still dark with raining clouds and the viewer is under clear sky in the direction of the sun. The result is a really bright rainbow contrasting with the dark background.
Supernumerary Rainbow (Stacker Rainbow): A very rare phenomenon consisting of several faint rainbows on the inside of the primary one. Their pattern doesn’t fit the usual color band and cannot be explained with classical geometric optics. They are caused by interference between rays of light, because the light follows slightly different paths varying in length. The very existence of supernumerary rainbows proves the wave nature of light and was exemplified and explained by Thomas Young in 1804, making them highly significant. An example of one: https://haplofromnexus.files.wordpress.com/2012/11/edit-600×398.jpg
Moonbow: Also known as a lunar rainbow is a rainbow produced by the light reflected off the moon, rather than direct sunlight. They are thus significantly faint and hard to see. Here is an example: https://haplofromnexus.files.wordpress.com/2012/11/moonbow.jpg
“A colored circle around the moon is not a moonbow—it is usually a 22° halo produced by refraction through hexagonal ice crystals in cirrus clouds”
Here are some links to read more about these subjects,
Mete.
Links:
http://eo.ucar.edu/rainbows/
http://www.atoptics.co.uk/rainbows/supers.htm
http://www.ratestogo.com/blog/moonbows/
HaplofromNexus 