Rainbows
A rainbow is one of our atmosphere’s most
exquisite and marvelous creations;
"one of the most spectacular light shows
observed on earth" (Ahrens,
1998). When a person views a rainbow, they
are getting a personal light show
that no other person can see as they do.
Humphreys points out that: "Since
the rainbow is a special distribution of
colors (produced in a particular way)
with reference to a definite point -
the eye of the observer - and as no single
distribution can be the same for
two separate points, it follows that two
observers do not, and cannot, see
the same rainbow." (Humphreys, 1929). Of
course, a camera lens will record an
image of a rainbow which can then be seen
my many people (Lynds, 1995). A
rainbow is essentially made up of seven
brilliant colors: red, orange,
yellow, green, blue, indigo, and violet;
moreover, it is comprised of many
coloration that the eyes cannot see. No
painter can manufacture the colors of
the rainbow, for they create color by
mixing, but no mixing will give red,
green, or purple. These are the colors of
the rainbow, though between the red
and the green an orange color is often seen
(Aristotle, 350 BC). To
understand how the awesome production of a rainbow
occurs is a feat of
physics and mathematics. The two most important ingredients
are light and
drops of water. The manner and position in which the light and
water droplets
transverse, and the reactions between the two, are complex
formulas of
nature. Additionally, certain circumstances make it possible for a
rainbow to
occur. You cannot have a rainbow without some source of light. It is
possible
to create an artificial rainbow utilizing artificial light, but for
this
paper, I will only use instances and circumstances of naturally
occurring
light which produce naturally occurring rainbows. The sun is
nature’s primary
source of rainbow creating light; however, occasionally, the
light of the moon
can produce a rainbow. For a lunar rainbow to occur the
conditions have to be
just right. The moon has to be a full moon, which only
happens once a month. The
light of a full moon is a faint light, so the lunar
rainbow will never be as
prominent as the solar rainbow. Additionally, the
full moon has to be either
rising or setting. A naval officer, V. E.
Mikkelson, described his reaction when
he learned that the spotlight he saw
was actually a lunar rainbow: The strange
phenomenon I was viewing was a
rainbow generated by moonlight! It was being
formed by moonlight passing
through rain trailing from one of the fluffy cumulus
clouds. The rainbow was
composed of the standard colors, but they were softer
and paler than those
generated by direct sunlight. They were what I can only
describe as pastel. I
was truly entranced. I have seen many of nature's
beauties, but none have
ever impressed me so powerfully (Mikkelson, 1994) This
is a rare phenomenon.
Captain Mikkelson has been a seaman for over twenty years
and has only viewed
this one lunar rainbow; moreover, he has spoken to numerous
career seamen and
none reported ever seeing such a magnificent sight. Another
essential
component of the rainbow is drops of water. Again, water drops
from
sprinklers or the sprays emitted from waterfalls can produce a rainbow,
but I
will focus on water drops that fall from the sky. A raindrop is
nature’s
reflective mirror in the sky. The smooth, yet internally curved,
surface of a
raindrop can refract and reflect light. A typical raindrop is
spherical in
dimension and thus has a symmetrical effect on light. Lynds
(1995) points out
that the "purity" of the colors of the rainbow depends on
the size of
the raindrops. Large drops (diameters of a few millimeters) give
bright rainbows
with well defined colors; small droplets (diameters of about
0.01 mm) produce
rainbows of overlapping colors that appear nearly white.
When it rains, the
droplets are never the same exact size and shape; thus, a
rainbow occurs in a
range of colors. Sunlight slows and bends when it enters
a raindrop. Some of the
light strikes the back of the drop at such an angle
that it is reflected within
the drop, this is the critical angle of 48º. Any
angle greater than this and
the light bounces off the back of the raindrop
and is internally reflected
towards our eyes. Each light ray bends
differently from the others, so each
angle is slightly different; therefore,
the light is dispersed into a spectrum
of colors from red, angle of 42º, to
violet, angle of 40º. This is how a
sundry of raindrops produce the dazzling
colors of a rainbow (Ahrens, 1998).
Rainbows never form a full circle or
any segment greater than a semicircle. At
sunset and sunrise, the circle is
smallest and the segment largest: as the sun
rises higher the circle is
larger and the segment smaller. Lynds (1995)
explains: "We do not see a full
circle because the earth gets in the way.
The lower the sun is to the
horizon, the more of the circle we see -right at
sunset; we would see a full
semicircle of the rainbow with the top of the arch
42 degrees above the
horizon. The higher the sun is in the sky, the smaller is
the arch of the
rainbow above the horizon." There are never more than two
rainbows at one
time. Each of them have three primary colors that blend and make
the full
visible spectrum of the rainbow. The colors are the same in both and
their
number and relation, but in the outer rainbow they are fainter and
their
position is reversed. In the inner rainbow the first and largest band
is red; in
the outer rainbow the band that is nearest to this one and
smallest is of the
same color: the other bands correspond on the same
principle. The rainbow we
normally see is called the primary rainbow and is
produced by one internal
reflection; the secondary rainbow arises from two
internal reflections. When
sight becomes strained to a great distance, the
appearance of the distant object
is affected; it appears fainter;
additionally, everything at a distance looks
blacker, because sight does not
reach it. The secondary, outer, rainbow is
further away and less of it
reaches the sun, so the reflection from the outer
rainbow is weaker. This is
why three rainbows or more would not be observed,
because even the second is
fainter, so that the third reflection would not have
enough strength and
would not reach the sun. The order of the secondary rainbow
is reversed
because more reflection reaches the sun from the smaller, inner
band. Because
the band that is nearest the primary, inner, will reflect and
result in the
same color band of the secondary rainbow. Now the smallest band in
the outer
rainbow is that which is nearest, and so it will be red; and the
second and
the third will follow the same principle (Aristotle, 350 BC). In the
shorter
days after the autumn equinox there may be a rainbow at any time of the
day,
but in the longer days from the spring to the autumn equinox there cannot
be
a rainbow about midday. The reason for this is that when the sun is north
of
the equator the visible arcs of its course are all greater than a
semicircle,
and go on increasing, while the invisible arc is small, but when
the sun is
south of the equator the visible arc is small and the invisible
arc great, and
the farther the sun moves south of the equator the greater is
the invisible arc
(Webster, 1994). Rainbows are observed more frequently in
the summer than the
winter because one has to have rain and sunshine. In the
winter, water droplets
freeze into ice particles that do not produce a
rainbow but scatter light in
other very interesting patterns (Humphreys,
1929). Proverbs about rainbows
predicting future weather have been around for
centuries. "Red sky at
night, sailors delight. Red sky in the morning, sailor
take warning",
"Rainbow at night, shepherd's delight; Rainbow in morning,
shepherds take
warning.", "If there be a rainbow in the eve, It will rain and
leave;
But if there be a rainbow in the morrow, it will neither lend nor
borrow",
and "Rainbow to windward, foul fall the day; rainbow to leeward,
damp runs
away" (Humphreys, 1923). The reason these proverbs are seemingly
logical
when you consider the fact that to see a rainbow rain must be falling
in part of
sky, and the sun shining in the other (Ahrens, 1998). In addition,
to see a
rainbow, or red sky, an individual has to be facing the rain and
have the sun at
your back. When we view an evening rainbow, we are facing
east and towards the
rain shower. When we view a morning rainbow, we are
facing west, consequently,
also towards the rain shower. In middle latitudes,
clouds and wind tend to
travel from west to east. Therefore, it follows that
the rain that is in the
west, as it is in a morning rainbow, will make its
way across the land,
eventually reaching our point of
observation.
Bibliography
Ahrens,D. C., (1998). Essentials of
meteorology (2nd ed.). Belmont, CA: West
Publishing House Aristotle, (350
B.C.E.), Meteorology (Translated by E. W.
Webster, 1994). [On-line].
Available: http://www.
classics.mit.edu/Aristotle/meteorology.3.iii.htm
Humphreys, W. J.,
(1929). Physics of the air. Europe: McGraw-Hill Book Co.
Humphreys, W.
J., (1923). Weather proverbs and paradoxes. Williams and Wilkins
Company.
Lynds, B. T., (1995, September 19). About rainbows. [On-line].
Available:
http://www.unidata.ucar.edu/staff/blynds/rnbw.html. Mikkelson, V. E.,
(1994).
Unexpected Beauty. [On-line]. Available:
http://www.unidata.ucar.edu/staff/blynds/Mikkelson.html