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.
What's In a Name?
Ever wondered where the word "rainbow" comes from? The "rain-" part of it stands for the rain drops required to make it, whereas "-bow" refers to its arc shape.
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 more dense that air). This causes light's path to bend or "refract."
Hold that thought! 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 it's range of colors) exits the water droplet, it speeds up as it travel 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. From the diagram in above pic, 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. I know what you're thinking..."Rainbows aren't circular -- they're a semi-circle." Right? 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 singe and double rainbows is that the color scheme for double rainbows is reversed. (It's 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 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.
Resources: NASA SciJinks. What Causes a Rainbow? Accessed 20 June 2015.NOAA National Weather Service Flagstaff, AZ. How do Rainbows Form? Accessed 20 June 2015.The University of Illinois Department of Atmospheric Sciences WW2010. Secondary Rainbows. Accessed 21 June 2015.
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