The rainbow is a beautiful natural phenomenon that has fascinated humans for millennia. It is formed when sunlight interacts with water droplets in the atmosphere, separating into the familiar spectrum of colors we see in the sky. But what exactly are the 7 colors of the rainbow in order? Here, we will examine the science behind rainbows and list the proper sequence of colors from top to bottom.
How Rainbows Form
Rainbows are optical illusions that occur when sunlight passes through water droplets suspended in the air. As white sunlight enters a droplet, it slows down and bends, a process called refraction. The different wavelengths of light – corresponding to different colors – bend at slightly different angles, causing the white light to fan out into the visible color spectrum.
The water droplets act like tiny prisms, refracting the sunlight into its constituent colors and sending it back out towards the viewer. The light bounces and reflects multiple times inside each droplet, a process called dispersion. This leads to the continuum of rainbow colors we can observe.
The Sequence of Rainbow Colors
The colors of the rainbow arise from the spectrum of visible light. Visible light ranges in wavelength from about 400 to 700 nanometers (nm) and spans the colors violet, indigo, blue, green, yellow, orange, and red.
When sunlight is dispersed by water droplets, the colors separate based on their wavelength. The longest wavelengths (red) bend the least, while the shortest wavelengths (violet) bend the most.
The 7 colors of the rainbow in their proper order from top to bottom are:
| Color | Wavelength (nm) |
| Red | 620-750 |
| Orange | 590-620 |
| Yellow | 570-590 |
| Green | 495-570 |
| Blue | 450-495 |
| Indigo | 445-450 |
| Violet | 380-445 |
The sequence is remembered using the acronym ROY G BIV, standing for the first letters of each color. Red has the longest wavelength and sits at the top of the rainbow arc, with violet having the shortest wavelength visible to humans.
Why 7 Colors?
Many sources list the rainbow as having 7 distinct colors, separated into the visible spectrum. This originated in the early 17th century, when philosopher René Descartes proposed that the rainbow is made up of 7 fundamental colors.
Though others before him had observed 5 or 6 colors, Descartes’ schema stuck and became convention. His influence at the time led many to adopt the 7 color model.
In reality, there are not precisely 7 separate colors in a rainbow. It is a continuous spectrum that our eyes and brain divide into categories. The number of discernible colors depends on how finely they are distinguished.
Some cite Newton’s work with prisms in the late 1600s, where he divided the spectrum into 5 principal colors: red, yellow, green, blue and violet. Later work expanded this to include orange and indigo.
So while there are not literally 7 bands of color, the 7 color model persists as a simple way to understand the visible spectrum in a rainbow. The key colors of red, orange, yellow, green, blue, indigo and violet represent the full arc reasonably well.
The Optics of Rainbows
Rainbows form when sunlight and rainwater meet under specific circumstances. For a rainbow to occur, the following is needed:
- Direct sunlight striking water droplets in the atmosphere.
- The sunlight must enter the droplets and reflect/refract inside them.
- The viewer’s back must be to the sun, with the rain/drops in front.
- Sunlight, raindrops and viewer must be at a 40-42 degree angle.
As sunlight passes from air into a water droplet, refraction causes it to bend and split into a spectrum of colors. When this light reflects off the back inner surface of the drop, more refraction occurs. The now separated colors fan out and emerge from the droplet, heading towards the viewer.
This whole process causes the spectrum to orient itself as an arc, with red on the top outer edge. The colors then align in their sequence based on wavelength as they move through the droplets.
For a viewer to see the rainbow, their line of sight must be at approximately a 40-42 degree angle from the direction of sunlight penetrating the rain shower. The arc of the rainbow will be centered on this anti-solar point, directly opposite the sun.
Double Rainbows
Sometimes a secondary, fainter rainbow can be observed outside the primary one. This is known as a double rainbow. It is formed by a different optical path and order of reflections inside the raindrops.
In a double rainbow:
- Sunlight enters a droplet and is reflected once off the back surface.
- Some of this reflected light is reflected again, making a total of 2 internal reflections.
- The twice-reflected light exits the droplet at an angle of about 50-53° from the sun.
This creates the secondary rainbow outside the brighter primary bow. The color order of the secondary rainbow is inverted, with red on the bottom instead of top.
Double rainbows reinforce that the rainbow is not separate bands of color, but a continuous spectrum. More reflections can form even more rainbows further outside the second one.
Rainbows in Nature
Rainbows can occur any time there is both sunlight and sufficient raindrops, spray, or mist to disperse the light. Typical rainbows form when sunlight intersects with rain, but other natural occurrences produce rainbow effects too, such as:
- Mist from waterfalls – Rainbows sometimes form in mist thrown up by powerful waterfalls.
- Spray from waves – Ocean spray stirred up around crashing waves can create miniature rainbows.
- Morning dew – Dew droplets on grass may generate small ground-level rainbows at dawn.
- Icicles – Sunlight shining through icicles can create faint rainbow patterns.
Any time water droplets or ice crystals suspended in air interact with sunlight at the right angle, some form of rainbow may become visible. The more pronounced rainbows appear when sunlight and rain mix.
Rainbows in Culture
Rainbows have carried meaning and symbolism throughout human history and across cultures. Here are some of the prevalent associations of rainbows:
- Hope, promise, dreams – The rainbow has long been seen as a sign of hope after hardship, probably from the Biblical story of Noah’s flood.
- Good fortune – In some cultures, rainbows represent good luck, fortune and positivity.
- Bridge between realms – Rainbows are seen as bridges, pathways or connections between worlds, the heavens and earth, or spirit realms.
- Chasers of rainbows – Pot of gold at the end, happiness, dreams yet to be fulfilled.
Rainbows often carry uplifting, optimistic symbolism, though they may have other connotations too, such as a warning or spirit encounter in some Native American tribes. However they are viewed, rainbows continue to inspire awe, joy and imagination across cultures.
Conclusion
In review, the proper sequence of the 7 main colors constituting a rainbow is red, orange, yellow, green, blue, indigo and violet. This visible color spectrum arises when sunlight interacts with water droplets in the atmosphere, becoming separated due to different wavelengths bending at slightly different angles.
Though not literally 7 distinct bands, the 7 color model captures the full rainbow arc reasonably well. The optics of rainbow formation depend on the precise angle between sunlight, raindrops and viewer. Rainbows also occur in nature outside of rain, whenever sunlight meets suspended water or ice droplets. These colorful optical phenomena carry deep meaning for many cultures as symbols of dreams, hope and inspiration.
References
– National Geographic – https://www.nationalgeographic.org/media/rainbows/
– Britannica – https://www.britannica.com/science/rainbow-optics
– Physics of Rainbows – https://www.physicsclassroom.com/class/refrn/Lesson-2/The-Physics-of-Rainbows
