Arch of Color: A Review on Rainbows

When you were younger, you probably learned about the acronym ROYGBIV in order to memorize the colors of the rainbow. You were probably told that rainbows were magical structures that would make you super lucky if you ever saw one–at least this was what I was told as a child. Sorry to burst your bubble but rainbows are actually just optical illusions that form based on certain weather conditions. The science behind how they are perceived is quite interesting.

Rainbows are formed when sunlight interacts with raindrops and then reflects back to the eyes of the observer. The ideal conditions for the observer to spot a rainbow is when a storm is nearing its end and the sun is peeking out from behind the rain clouds. This will allow sunlight to interact with the droplets of rain. The position of the observer must also follow these conditions:

• Sun should be behind the observer

• Sun should be low in the sky, at an angle of less than 42 degrees above horizon

• Source of droplets should be in front of the observer
  

How does sunlight and water droplets create such a vibrant display of colors? Imagine shining a beam of light through a clear triangular prism: you’d see white light transform into a dazzling spectrum of colors. This happens because the glass of the prism is denser than air. As white light enters the prism, it slows down and bends—a process known as refraction. This bending causes the different wavelengths of visible light to spread out and reveal their distinct colors.

Source: Kelvin Biggs/UQG Optics

Let’s talk about visible light. Visible light is a range of wavelengths in the electromagnetic spectrum that the human eye can see.  It spans a range of wavelengths and includes the colors red, orange, yellow, green, blue, indigo, and violet. Each color corresponds to a specific wavelength, with red having the longest and violet the shortest. When all these colors come together, they form white light. This is why, when white light is split, it reveals the full spectrum of colors.

The interaction between a prism and white light mirrors the way sunlight interacts with raindrops to create a rainbow. Contrary to popular belief, raindrops are not tear-shaped but are actually spherical. This spherical shape is crucial for rainbow formation. Because water is denser than air, the sunlight, which is white light, refracts within the droplet. This process of refraction causes the white light to split into the visible light spectrum. Because of the droplet’s spherical shape, light undergoes refraction three times: first as it enters the droplet, then as it reflects off the inner surface, and finally as it exits back into the air. Each time the light is refracted, the split between each of the wavelengths widens because each component wavelength of sunlight refracts by a slight difference in amount. Since each component bends at a different angle, it widens the array of colors.

Source: Met Office

The amount of bending varies by color: red light, with the longest wavelength and lowest frequency, refracts the least and thus appears on the outer edge of the rainbow. Conversely, violet light, with the shortest wavelength and highest frequency, refracts the most and forms the innermost band. This process creates the vivid spectrum of colors seen in a rainbow.

Fun Fact: While each droplet does split sunlight into the full visible spectrum, only one color from the spectrum can actually be seen in each raindrop. This makes it so everyone sees their own unique rainbow, created by different sets of water droplets!

There are various different kinds of rainbows: Double rainbows, Moonbows, and Fogbows. Double rainbows are created when light is reflected twice inside the raindrops. This causes a secondary rainbow  to sit above the primary rainbow. However, the interesting part is that the secondary rainbow’s colors are reversed. So, violet forms the outermost band and red forms the innermost band. Moonbows are rainbows that form from the light of the moon. Since the moon’s light is much dimmer than the light of the sun, these moonbows are relatively more faint than normal sunlight rainbows. Fogbows, also known as ghost rainbows, are rainbows that appear to look cloudy or white. These form when the water droplets are far too small for light to refract in them. This is why these types of rainbows only really occur when it is misty or foggy.

Ultimately, rainbows are just made out of sunlight. While they are not magical, rainbows are still a unique and interesting weather phenomenon. Whenever the sun comes out after a rainstorm, remember to follow the rainbow sightseeing conditions to spot one! I hope you learned something new! Keep a lookout for the next post!

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References

Means, T. (2021 July 19). How Are Rainbows Formed? Overview and Ideal Conditions. Treehugger. Retrieved March 20, 2023, from https://www.treehugger.com/how-are-rainbows-formed-5191060

Met Office. (n.d.). How are rainbows formed?. Met Office. Retrieved March 20, 2023, from https://www.metoffice.gov.uk/weather/learn-about/weather/optical-effects/rainbows/how-are-rainbows-formed

Water Science School. (2018 June 8). Rainbows (Water and Light). USGS. Retrieved March 20, 2023, from https://www.usgs.gov/special-topics/water-science-school/science/rainbows-water-and-light