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Sun and Rain : A Recipe
For Rainbows
.
Rainbows are basically sunlight spread out
into its spectrum of colors – it is an optical phenomenon - it isn't something
that can be touched or that exists in a particular place. As the light rays from the sun strike and enter
a water droplet, their speed slows down a bit. This causes light's path to bend
or "refract." When any of these things happen, the different color
wavelengths are separated and can each be seen. So, when a ray of
light enters a raindrop and bends, it separates into its component color
wavelengths. As
long as there's bright sunlight, suspended water droplets, and you're
positioned at the proper viewing angle, it's possible a rainbow could be within
view! Whether
you believe they're a sign of God's promise, or there's a pot of gold waiting
for you at their end, rainbows are one of nature's most happy-inducing
displays.
By Tiffany Means
Whether you believe they're a
sign of God's promise, or there's a pot of gold waiting for you at their end,
rainbows are one of nature's most happy-inducing displays.
Why do we so rarely see
rainbows?
And why are they here one
minute and gone the next? Click on to explore the answers to these and other
rainbow-related questions.
What Is a Rainbow?
Because a rainbow is an
optical phenomenon (for you sci-fi fans, that's sort of like a hologram) it
isn't something that can be touched or that exists in a particular place.
Ever wondered where the word
"rainbow" comes from?
The "rain-" part of
it stands for the raindrops required to make it, whereas "-bow"
refers to its arc shape.
What Ingredients Are Needed to Make a Rainbow?
Rainbows tend to pop up during a sunshower (rain
and sun at the same time) so if you guessed sun and rain are two key
ingredients to making a rainbow, you're correct.
Rainbows form when the following conditions come
together:
The sun is behind the observer's position and is
no more than 42° above the horizon
It's raining in front of the observer
Water droplets are floating in the air (this is
why we see rainbows right after it rains)
The sky is clear enough of clouds for the
rainbow to be seen.
The Role of Raindrops
The rainbow-making process begins when sunlight
shines on a raindrop.
As the light rays from the sun strike and enter
a water droplet, their speed slows down a bit (because water is denser than
air). This causes light's path to bend or "refract."
Before we go any further, let's mention a few
things about light:
Visible light is made up of different color
wavelengths (which appear white when mixed together)
Light travels in a straight line unless
something reflects it, bends (refracts) it, or scatters it.
When any of these things happen, the different
color wavelengths are separated and can each be seen.
So, when a ray of light enters a raindrop and
bends, it separates into its component color wavelengths.
The light continues traveling through the drop
until it bounces (reflects) off the back of the droplet and exits the opposite
side of it at a 42° angle.
As the light (still separated into its range of
colors) exits the water droplet, it speeds up as it travels back out into the
less dense air and is refracted (a second time) downward to one's eyes.
Apply this process to a whole collection of
raindrops in the sky and voilá, you get an entire rainbow.
Why Rainbows Follow ROYGBIV
Ever noticed how a rainbow's colors (from
outside edge to inside) always go red, orange, yellow, green, blue, indigo,
violet?
In a previous diagram, we see that red light
refracts out of the water droplet at steeper angles to the ground.
So, when one looks at a steep angle, the red
light from the higher drops travels at the correct angle to meet one's eyes.
(The other color wavelengths exit these drops at
more shallow angles, and thus, pass overhead.)
This is why red appears at the top of a rainbow.
Now consider the lower raindrops.
When gazing at shallower angles, all droplets
within this line of sight direct violet light to one's eye, while the red light
is directed out of the peripheral vision and downward at one's feet.
This is why the color violet appears at the
rainbow's bottom.
The raindrops in-between these two levels bounce
different colors of light (in order from next longest to next shortest
wavelength, top to bottom) so an observer sees the full-color spectrum.
Are Rainbows Really Bow-Shaped?
Since raindrops are relatively circular in
shape, the reflection they create is also curved.
Believe it or not, a full rainbow is actually a
full circle, only we don't see the other half of it because the ground gets in
the way.
The lower the sun is to the horizon, the more of
the full circle we're able to see.
Airplanes offer a full view since an observer
could look both upward and downward to see the complete circular bow.
Double Rainbows
A few slides ago we learned how light goes
through a three-step journey (refraction, reflection, refraction) inside of a
raindrop to form a primary rainbow.
This "re-reflected" light exits the
drop at a different angle (50° instead of 42°) resulting in a secondary rainbow
which appears above the primary bow.
Because light undergoes two reflections inside
the raindrop, and fewer rays go through the 4-step it's intensity is reduced by
that second reflection and as a result, it's colors aren't as bright.
Another difference between single and double
rainbows is that the color scheme for double rainbows is reversed. (Its colors
go violet, indigo, blue, green, yellow, orange, red.)
This is because violet light from higher
raindrops enters one's eyes, while red light from the same drop passes over
one's head.
At the same time, red light from lower raindrops
enters one's eyes and the red light from these drops is directed at one's feet
and isn't seen.
And that dark band in-between the two arcs? It's
a result of differing angles of reflection of light through the water droplets.
(Meteorologists call it Alexander's dark band.)
Triple Rainbows
In the spring of 2015, social media lit up when
a Glen Cove, NY resident shared a mobile photo of what appeared to be a
quadruple rainbow.
Not only would it require multiple reflections
within the raindrop, but each iteration would produce a fainter bow, which
would make tertiary and quarternary rainbows quite hard to see.
When they do form, triple rainbows are typically
seen up against the inside of the primary arc (as seen in the photo above), or
as a small connecting arc between the primary and secondary.
Rainbows Not in the Sky
Rainbows aren't only seen in the sky. A backyard
water sprinkler. Mist at the base of a splashing waterfall.
As long as there's bright sunlight, suspended
water droplets, and you're positioned at the proper viewing angle, it's
possible a rainbow could be within view!
It's also possible to create a rainbow without
involving water. Holding a crystal prism up to a sunny window is one such
example.
Tiffany Means
Tiffany Means
Meteorology
Expert
Education
B.S.,
Atmospheric Sciences and Meteorology, University of North Carolina
Introduction
Studied
atmospheric sciences and meteorology at the University of North Carolina
Former
administrative assistant for the National Oceanic and Atmospheric
Administration
Member
of the American Meteorological Society
Experience
Tiffany
Means is a former writer for ThoughtCo who contributed articles about weather
for five years. She has interned with the domestic and international weather
departments at CNN, written monthly climate reports for NOAA’s National Centers
for Environmental Prediction, and participated in a number of science outreach
events, including the Science Olympiad Competition. Means has personally
experienced such weather greats as the Blizzard of 1993 and the floods of
Hurricane Francis (2004) and Ivan (2004).
Education
Bachelor's
degree in atmospheric sciences and meteorology from the University of North
Carolina at Asheville
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https://www.thoughtco.com/sun-rain-rainbows-3444159
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