The rainbow is a beautiful natural phenomenon that has inspired wonder and curiosity throughout human history. But how many colors make up a rainbow exactly? While many of us learn there are 7 colors of the rainbow in school, the full spectrum of colors in a rainbow is made up of 12 distinct bands.
Rainbows are optical light phenomena that occur when sunlight interacts with water droplets in the atmosphere. The light enters the droplet, slows down and bends, reflects off the back of the droplet, and exits at a different angle. This process separates sunlight into its composite wavelengths, revealing the spectrum of colors that make up white light.
To the human eye, a rainbow appears as an arched multi-colored band across the sky. But when observed and measured scientifically, it becomes clear that the rainbow contains many more than 7 distinct bands of color. In reality, there are at least 12 discernible wavelengths of color that complete the full rainbow spectrum.
How Rainbows Form
Rainbows occur when three conditions are met:
- There must be water droplets or mist in the air, such as after a rainfall.
- The sun must be shining from behind the observer onto the water droplets.
- The observer must be facing their back to the sun’s general direction.
As sunlight enters a water droplet, some light is reflected off the front and back surfaces of the droplet. However, much of the light enters and slows down as it passes through the denser water. This causes the light to bend or refract. When the refracted light inside the droplet reaches a critical angle, it reflects completely off the back interior surface of the droplet. This reflected light then exits the droplet at an angle of 40-42 degrees relative to the incoming light.
Because the different wavelengths (colors) of sunlight bend at slightly different angles as they pass through the water, they become separated and spread out into a continuous band or spectrum of colors as they emerge from the droplets. The light at the top of the rainbow’s arch emerges at an angle of 40 degrees, while the bottom of the arch emerges at 42 degrees.
The 12 Rainbow Colors
When sunlight separates into its composite wavelengths, the colors we see in a rainbow emerge in the following order from top to bottom of the arc:
- Red
- Orange
- Yellow
- Chartreuse green
- Green
- Blue-green
- Cyan
- Azure (blue)
- Violet
- Purple
- Magenta
- Red
This sequence of 12 rainbow colors remains constant and unchanging. However, the human eye and optics of the atmosphere often blend these colors together into the 7 bands of color most commonly associated with rainbows.
Red
Red is the first and last color of the rainbow spectrum. It has the longest wavelength visible to the human eye. Red light refracts the least passing through a water droplet, exiting at a critical angle of 40 degrees relative to the incoming sunlight.
Orange
Orange is the second color and lies between red and yellow along the rainbow. Its wavelength range overlaps with red on one side and yellow on the other.
Yellow
Yellow light has a medium-length wavelength. Sunlight in this range stimulates the cone cells in our eyes most strongly, causing yellow to appear very bright to us.
Chartreuse
The greenish-yellow color known as chartreuse lies between yellow and green along the rainbow spectrum. Most observers blend chartreuse together with the yellow and green bands.
Green
Green is at the center of the visible color spectrum. Its wavelength stimulates both the medium and short cone color receptors in the human eye. This mid-range wavelength bends at an intermediate angle passing through water droplets.
Blue-Green
On the rainbow, blue-green is the transitional color between green and blue. It also overlaps with the following cyan band.
Cyan
Cyan appears as a distinctly blue-green color midway down the rainbow arc. While it overlaps with blue-green and green wavelengths, cyan strongly stimulates the short cone color receptors in our eyes.
Azure
The azure band is a deep blue color situated between cyan and violet. This is the blue most people associate with rainbows.
Violet
Violet light has a shorter wavelength than any other visible color. As it passes through a water droplet, it refracts and reflects at a critical angle of about 42 degrees.
Purple
Purple blends the short wavelengths of violet and blue light with longer wavelengths of red light. Most observers will see violet and purple as a single blended band of color when looking at a rainbow.
Magenta
Magenta sits between purple and red near the bottom end of the rainbow spectrum. This color stimulates both the long and short wavelength color receptors in our eyes almost equally.
Why We Traditionally See 7 Colors
If rainbows contain 12 distinct bands of color, why do we traditionally think of rainbows as having only 7 colors? There are a few reasons for this:
- The human eye has difficulty discerning the subtle color distinctions within the spectrum at a glance.
- Adjacent wavelengths in the rainbow tend to visually blend together.
- The common RYGBV color sequence contains 7 letters.
- Cultural tradition and teaching reinforces the 7 color sequence.
However, with careful observation and optics, it’s clear that rainbows consistently display the 12 color bands described above. The classic RYGBV 7-color sequence groups the main rainbow colors together while omitting transitional shades like chartreuse, cyan, azure, and magenta.
Viewing the Full Rainbow Spectrum
You can train your eye to distinguish all 12 rainbow colors by viewing rainbows under ideal conditions. Here are some tips:
- Look for vivid primary and secondary bows shortly after rain when the air is still humid.
- Face away from the sun with your back directed at its general position.
- Observe bright rainbows that contrast well against a dark background.
- Pick out boundary colors first, then look for transitional bands.
- Capture photos of rainbows and zoom in to see color banding.
- Use a diffraction grating to split sunlight into a rainbow spectrum.
Recreating the Rainbow
You can recreate the full rainbow spectrum at home using a glass prism or diffraction grating. Here’s how:
- Face a prism towards a window or other bright sunlight source.
- Place a white screen or sheet of paper opposite the prism to catch the refracted light.
- Rotate the prism to direct the spectrum onto the screen.
- View the rainbow colors starting with red bending the least, and violet bending the most.
With good sunlight, you should be able to distinguish all 12 rainbow colors. This demonstrates the innate separation of wavelengths that occurs in rainbows.
Significance of the 12 Colors
Appreciating all 12 rainbow colors has value both scientifically and artistically:
- It reveals the wondrous natural physics of light interacting with water.
- It represents the full separation of visible wavelengths in nature.
- It provides a complete palette for using rainbow colors artistically.
- It allows us to fully enjoy the optical beauty of rainbows.
- It encourages closer observation and appreciation of our natural world.
So the next time you encounter one of nature’s rainbows, take an extra moment to see if you can discern all 12 colors from top to bottom. You’ll gain a deeper appreciation of the optical phenomenon that gives rainbows their magical allure.
Conclusion
While rainbows are commonly represented as having 7 distinct bands of color, scientifically there are 12 complete wavelengths that comprise a full rainbow spectrum. By understanding the optical process that separates sunlight into a rainbow and carefully observing rainbows under ideal conditions, anyone can learn to discern the 12 true colors of the rainbow.
Taking the time to fully observe the spectrum reveals nature’s wonderful interplay of light, water and vision. Appreciating all 12 bands of color grants a richer recognition of rainbows’ delicate beauty and inspires curiosity about the natural world around us.
