A rainbow
is an optical and meteorological
phenomenon that is caused by reflection of light in water droplets in the
Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It
takes the form of a multicoloured arc.
Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.
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 while 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, red facing toward the other one, in both rainbows.
This second rainbow is caused by light reflecting twice inside water droplets.
Overview
The
rainbow is not located at a specific distance, but comes from any water
droplets viewed from a certain angle relative to the Sun's rays. Thus, a
rainbow is not an object, and cannot be physically approached. Indeed, it is
impossible for an observer to manoeuvre to see any rainbow from water droplets
at any angle other than the customary one of 42 degrees from the direction
opposite the Sun. 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 further off-yet, at the same angle as seen by the
first observer. A rainbow spans 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 a normal human eye, the most
commonly cited and remembered sequence is Newton's sevenfold red, orange,
yellow, green, blue, indigo and violet.
Rainbows can be caused by many forms of airborne
water. These include not only rain, but also mist, spray, and airborne dew.
Rainbows can form in mist, such as that of a waterfall
Rainbow with a faint reflected
rainbow in the lake
Rainbows may form in the spray
created by waves (called spray bows)
Rainbow after sunlight bursts through after an intense shower in Maraetai, New Zealand
Circular rainbow seen while skydiving over Rochelle, Illinois
Visibility
Rainbows
can be observed whenever there
are water drops in the air and sunlight shining from behind at a low altitude
angle. The most spectacular rainbow displays happen when half the sky is still
dark with raining clouds and the observer is at a spot with clear sky in the
direction of the sun. The result is a luminous rainbow that contrasts with the
darkened background.
The rainbow effect is also commonly seen near waterfalls or fountains. In addition, the effect can be artificially created by dispersing water droplets into the air during a sunny day. Rarely, a moonbow, lunar rainbow or nighttime rainbow, can be seen on strongly moonlit nights. As human visual perception for colour is poor in low light, moonbows are often perceived to be white. It is difficult to photograph the complete semicircle of a rainbow in one frame, as this would require an angle of view of 84°. For a 35 mm camera, a lens with a focal length of 19 mm or less wide-angle lens would be required. Now that powerful software for stitching several images into a panorama is available, images of the entire arc and even secondary arcs can be created fairly easily from a series of overlapping frames. From an aeroplane, one has the opportunity to see the whole circle of the rainbow, with the plane's shadow in the centre.
Number of colours in spectrum or rainbow
A
spectrum obtained using a glass prism and a point source, is a continuum of
wavelengths without bands. The
number of colours that the human eye is able to distinguish in a spectrum is in
the order of 100. Accordingly, the Munsell colour system (a 20th
century system for numerically describing colours, based on equal steps for
human visual perception) distinguishes 100 hues. However, the human brain tends
to divide them into a small number of primary colours. The apparent
discreteness of primary colours is an artefact of the human brain. Newton
originally (1672) divided the spectrum into five primary colours: red, yellow,
green, blue and violet. Later he included orange and indigo, giving seven
primary colours by analogy to the number of notes in a musical scale.
The Munsell colour system removed orange and indigo again, and returned to five
primary colours. The exact number of primary colours for humans is a somewhat
arbitrary choice.
Variations
Multiple rainbows
Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and appear at an angle of 50–53°. As a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow, with blue on the outside and red on the inside. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.
A double
rainbow features reversed colours in the outer (secondary) bow, with the dark Alexander's band between the bows.
Monochrome rainbow
Occasionally
a shower may happen at sunrise or sunset, where the shorter wavelengths like
blue and green have been scattered and essentially removed from the spectrum.
Further scattering may occur due to the rain, and the result can be the rare
and dramatic monochrome rainbow.
Fogbow
Fogbows
form in the same way as rainbows, but they are formed by much smaller cloud and
fog droplets which diffract light extensively. They are almost white with faint
reds on the outside and blues inside. The colours are dim because the bow in
each colour is very broad and the colours overlap. Fogbows are commonly seen
over water when air in contact with the cooler water is chilled, but they can
be found anywhere if the fog is thin enough for the sun to shine through and
the sun is fairly bright. They are very large—almost as big as a rainbow and
much broader. They sometimes appear with a glory at the bow's centre
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