Other attributes
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.
In a primary rainbow, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.
In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colours reversed, with red on the inner side of the arc. This is caused by the light being reflected twice on the inside of the droplet before leaving it.
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.
Image of the end of a rainbow at Jasper National Park
A rainbow is not located at a specific distance from the observer, but comes from an optical illusion caused by any water droplets viewed from a certain angle relative to a light source. Thus, a rainbow is not an object and cannot be physically approached. Indeed, it is impossible for an observer to see a rainbow from water droplets at any angle other than the customary one of 42 degrees from the direction opposite the light source. Even if an observer sees another observer who seems "under" or "at the end of" a rainbow, the second observer will see a different rainbow—farther off—at the same angle as seen by the first observer.
Rainbows span a continuous spectrum of colours. Any distinct bands perceived are an artefact of human colour vision, and no banding of any type is seen in a black-and-white photo of a rainbow, only a smooth gradation of intensity to a maximum, then fading towards the other side. For colours seen by the human eye, the most commonly cited and remembered sequence is Isaac Newton's sevenfold red, orange, yellow, green, blue, indigo and violet, remembered by the mnemonic Richard Of York Gave Battle In Vain, or as the name of a fictional person (Roy G. Biv). The initialism is sometimes referred to in reverse order, as VIBGYOR.
Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew.
- Glow
The atmosphere opposite a rainbow, facing the sun, is often glowing. This glow appears when rain or drizzle is falling between the viewer and the sun.
The glow is formed by light passing through raindrops, not reflected by them. Some scientists call this glow a zero-order glow.
- Double Rainbow
Sometimes, a viewer may see a "double rainbow." In this phenomenon, a faint, secondary rainbow appears above the primary one.
Double rainbows are caused by light being reflected twice inside the raindrop. As a result of this second reflection, the spectrum of the secondary rainbow is reversed: red is on the inner section of the arch, while violet is on the outside.
- Higher-Order Rainbows
Light can be reflected from many angles inside the raindrop. A rainbow's "order" is its reflective number. (Primary rainbows are first-order rainbows, while secondary rainbows are second-order rainbows.) Higher-order rainbows appear to viewers facing both toward and away from the sun.
A tertiary rainbow, for example, appears to a viewer facing the sun. Tertiary rainbows are third-order rainbows—the third reflection of light. Their spectrum is the same as the primary rainbow.
Tertiary rainbows are difficult to see for three main reasons. First, the viewer is looking toward the sun—the center of a tertiary rainbow is not the antisolar point, it's the sun itself. Second, tertiary rainbows are much, much fainter than primary or secondary rainbows. Finally, tertiary rainbows are much, much broader than primary and secondary rainbows.
Quaternary rainbows are fourth-order rainbows, and also appear to viewers facing the sun. They are even fainter and broader than tertiary rainbows.
Beyond quaternary rainbows, higher-order rainbows are named by their reflective number, or order. In the lab, scientists have detected a 200th-order rainbow.
- Twinned Rainbow
A twinned rainbow is two distinct rainbows produced from a single endpoint. Twinned rainbows are the result of light hitting an air mass with different sizes and shapes of water droplets—usually a raincloud with different sizes and shapes of raindrops.
- Supernumerary Rainbow
A supernumerary rainbow is a thin, pastel-colored arc usually appearing below the inner arch of a rainbow. Supernumeraries are the result of the complex interaction of light rays in an air mass with small, similarly sized water droplets.
In supernumerary formation, reflected rays interact in ways called constructive and destructive interference. Light is either reinforced (constructive interference) or canceled out (destructive interference). Interference is responsible for the lighter hues and narrower bands of supernumeraries.
- Reflection Rainbow
A reflection rainbow appears above a body of water. A primary rainbow is reflected by the water, and the reflected light produces a reflection rainbow. Reflection rainbows do not mirror the primary rainbow—they often appear to stretch above it.
Reflected Rainbow
A reflected rainbow appears directly on the surface of a body of water. A reflected rainbow is created by rays of light reflected by the water surface, after the rays have have passed through water droplets. Reflected rainbows to not appear to form a circle with a primary rainbow, although their endpoints seem to meet in an almond-shaped formation.
Red Rainbow
A red rainbow, also called a monochrome rainbow, usually appears at sunrise or sunset. During this time, sunlight travels further in the atmosphere, and shorter wavelengths (blue and violet) have been scattered. Only the long-wavelength red colors are visible in this rainbow.
Fogbow
A fogbow is formed in much the same way as a primary rainbow. Light in a fogbow is refracted and reflected by fog (water droplets suspended in air). A fogbow seen in the clouds is called a cloudbow.
Because the water droplets in fog are much smaller than raindrops, fogbows have much fainter colors than rainbows. In fact, some fogbows have few detectable colors at all and appear mostly white, with a reddish tinge on their outer edge and a bluish tinge on their inner edge.
Moonbow
A moonbow, also called a lunar rainbow, is a rainbow produced by light reflected by the Moon.
The Moon itself does not emit light, of course. Moonlight is reflected sunlight, as well as some starlight and "Earthlight." Because moonlight is so much fainter than sunlight, moonbows are dimmer than rainbows.
When can you see a rainbow?
A rainbow requires water droplets to be floating in the air. That’s why we see them right after it rains. The Sun must be behind you and the clouds cleared away from the Sun for the rainbow to appear.
