Rainbow

One of the most beautiful sights in the natural world is a rainbow -- a glistening arch of lustrous, metallic light and color that bends across the sky in front of a bank of dark storm clouds. Often, a second, dimmer rainbow arcs above the first, with a dark band between. Rainbows can also form in the spray of waterfalls, the spray blown from the tops of waves on the ocean, or even, in a fragmentary form, in the mist from a sprinkler or garden hose. The largest and most vivid rainbows, however, are those which form after rain.

Rainbows seem as if they occupy a specific point in space, like a triumphal arch, but, in fact, their apparent position is only the area of droplets that are sending their prismatic refractions towards to point where the observer is standing. Thus, if the observer actually moved towards the rainbow, even with rapid, perfect flight, they could not 'reach' it. The rainbow would move and ultimately disappear as the observer approached it. This does not detract from its beauty, of course -- in fact, it makes it even more visionary and ethereal.

How a rainbow is created

A rainbow occurs when sunlight bounces off millions of airborne drops of water after a rainstorm (or in the other circumstances described above). The drops of water refract the light, splitting it like a prism and reflecting most of it back sunwards. Because of this, it is possible for a partial rainbow to exist, since the arch is centered on the position of the viewer's eyes, and thus, if water droplets only cover part of the view, the parts of the arch which correspond to this area of air will be visible.

The reason a rainbow is multicolored -- showing all the primary colors of the spectrum when it is bright enough, in fact -- is that each color of light is of a different wavelength and refracts from the inside of the droplets at a different angle. Thus, the colors aren't 'mixed' because the angle of light reaching the viewer's eye from each section of the rainbow is reaching it at a specific angle.

If, for example, red light has an angle of 42 degrees, then the light refracting from drops at a 42 degree angle to the observer will appear red, while the other colors of light refracting from those drops travels at a different angle and does not reach the observer's eye at all. Thus, the position of the colored bands in the rainbow corresponds to the angle at which the light is refracted from the drops at that point, relative to the observer.

The sun must be at a low angle in order for a rainbow to be visible. The sun must be at 42 degrees above the horizon or less in order for the rainbow to appear; when the sun is higher, the 'position' of the rainbow is below the horizon and thus the observer cannot see the refracted spectrum of light.

Rainbow variations

Within this basic pattern, there is plenty of room for variety -- secondary rainbows, reflection rainbows, and the like. Secondary rainbows are larger, less brilliant bows surrounding the main rainbow, with a dark space called Alexander's band lying between them. The colors in these rainbows are arranged in reverse order to the main rainbow, the rainbow is wider, and its brightness is only one tenth that of the primary rainbow. Secondary rainbows are caused by the reflection of light at a different angle -- generally, 10 degrees outside the angle of the primary rainbow. There can be up to three secondary rainbows around a rainbow.

Reflection rainbows are oddly-shaped rainbows whose lower ends touch the primary rainbow, but which arch upwards at a sharper angle, crossing through the space usually occupied by Alexander's band. They are caused by a double reflection -- light from the sun reflecting upwards from the surface of a body of water onto the sheet of airborne drops that is creating the rainbow, and then refracting back towards the observer. The water from which the sunlight is being reflected up onto the falling rain must be located behind the observer in order for the whole reflection rainbow to be visible.

Alexander's band is an area of apparently darker sky between a primary and secondary rainbow. This effect has been known since 200 B.C., although it has only recently been explained. The reason for this dark band is that the drops between the two arcs lie at such an angle to the observer that none of their light reaches the observer, and thus, they are effectively black, darkening the space between the bows (although it is not completely dark, since light from beyond the drops is still reaching the observer's eye.

As can be seen, a rainbow is a complex and interesting weather phenomenon as well as an esthetically appealing one. It has no real effect on the weather, but it does give an interesting view of the workings of optics and light, and brings a touch of magic to the passing of a rainstorm.