SUN AND RAIN : A RECIPE FOR RAINBOWS - Rainbows are basically sunlight spread out into its spectrum of colors for us to see. 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. Rainbows tend to pop up during a sunshower 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 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." 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.

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Sun and Rain : A Recipe For Rainbows

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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?

Rainbows are basically sunlight spread out into its spectrum of colors for us to see.

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?

To find out why this is, let's consider raindrops at two levels, one above the other.

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?

We now know how rainbows form, but how about where they get their bow shape?

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.

But sometimes, light hits the back of a raindrop twice instead of just once.

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.

While possible in theory, triple and quadruple rainbows are extremely rare.

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.

These are all ways you can spot a rainbow.

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

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

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

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https://www.thoughtco.com/sun-rain-rainbows-3444159

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