The common belief is that rainbows consist of 7 main colors – red, orange, yellow, green, blue, indigo, and violet. This stems from Newton’s experiments with prisms in the 17th century, where he identified these 7 colors of the visible light spectrum. However, modern science tells us rainbows actually contain infinite colors, more than the 7 colors we traditionally think of.
The Origin of the 7 Rainbow Colors
In 1666, Isaac Newton conducted experiments passing sunlight through a prism. This separated the light into the color spectrum from red to violet that we are familiar with today. Newton decided there were 7 main colors in this spectrum, matching them to the 7 notes of the music scale. This idea of 7 rainbow colors stuck and became widely accepted.
Later scientists created the color wheel showing the 7 rainbow colors in order. This color wheel is still used in art, design, and color theory today. So while Newton’s 7 colors were influential, we now know the actual physics of rainbows is more complex.
Why Rainbows Have All Colors
Rainbows are created by the refraction and reflection of sunlight through water droplets in the air. As sunlight enters a raindrop, it is refracted, reflected internally, then refracted again as it exits the drop. This process separates the white sunlight into the full color spectrum.
The light leaving the raindrop spreads out in a cone shape, with each color bending at a slightly different angle based on its wavelength. Red light bends the least while violet light bends the most. This projects the spectrum of colors in an arc across the sky for us to see.
Importantly, there are no neat divisions or boundaries between the colors. Rainbows contain a continuous gradation of colors from red to violet, including every wavelength of visible light. There are infinitely many colors between what we call red, orange, yellow, etc. So rainbows aren’t limited to 7 colors.
The Visible Color Spectrum
The full visible spectrum of light that the human eye can see extends from wavelengths of about 380-750 nanometers. This range encompasses all the colors we perceive from violet (shortest wavelength) to red (longest wavelength). The table below shows the approximate wavelength ranges for different colors in the visible spectrum:
| Color | Wavelength range (nm) |
|---|---|
| Violet | 380-450 |
| Blue | 450-495 |
| Green | 495-570 |
| Yellow | 570-590 |
| Orange | 590-620 |
| Red | 620-750 |
As the table shows, there are no clear cut-offs between color categories. For example, there is no single wavelength where blue suddenly changes to green. Instead, wavelengths smoothly transition along the spectrum.
The Distribution of Colors in Rainbows
Because rainbows contain the full visible color spectrum, the distribution and intensity of colors we see follows the wavelength-dependence of sunlight. Shorter violet wavelengths are less abundant in sunlight compared to longer red wavelengths.
When sunlight first enters a raindrop, all wavelengths have equal intensity. But as light internally reflects and refracts through the drop, the probability of light escaping depends on its wavelength. Shorter wavelengths are more likely to remain trapped inside. This filters sunlight so longer wavelengths predominate by the time it exits in a rainbow.
Due to this physical process, rainbows appear predominantly yellow, orange, and red since sunlight contains more intensities at these longer wavelengths. Green, blue, and violet are still present but at lower intensities. So rainbows exhibit a continuous distribution peaking at long wavelengths rather than equal bands of color.
Rainbows from White Light vs. Sunlight
An interesting phenomenon occurs if you create a rainbow using a prism and white light rather than sunlight. With white light, which contains near-equal intensities across the visible spectrum, the resulting rainbow shows much more vivid blues, greens, and violets.
This is because sunlight’s natural wavelength distribution is filtered out. The rainbow appears more evenly distributed across colors since all wavelengths are equally represented in the original white light. This again demonstrates that rainbows contain a continuous spectrum, not discrete bands.
Total Internal Reflection Creates Rainbows
The fundamental physics behind rainbow formation is total internal reflection inside each raindrop. As light enters the drop, it refracts and bends. Upon hitting the back interior surface of the drop, light reflects if its incident angle exceeds a critical angle that depends on the refractive indices of water and air.
This reflected light propagates along the drop surface, undergoing total internal reflection. When it reaches the exit location, the light refracts again into the original incident cone angles relative to the entry point. This projects the spectrum of colors we observe in a rainbow.
Notably, the critical angle for total internal reflection is different for different wavelengths. So a cone of light exiting the raindrop contains a continuum of colors, rather than discrete bands at certain angles.
Double Rainbows
Sometimes we see an additional, fainter rainbow above the main rainbow arc. This is known as a double rainbow. It is caused by light undergoing a second internal reflection inside the raindrop before exiting.
The second reflection inverts the order of colors. The main rainbow’s order of outside to inside is red, orange, yellow, green, blue, indigo, violet. But the second rainbow appears as violet, indigo, blue, green, yellow, orange, red.
Again, the key point is that both rainbows contain a continuous distribution of spectral colors, not just 7 distinct bands.
Rainbows in Nature
Rainbows can be created by any small water droplets in the air, not just rain. Sunlight refracting through mist, fog, spray from a waterfall or sprinkler will also generate rainbows. Interestingly, observations and photographs show that double or even triple rainbows occur commonly in nature given the right conditions.
Nature provides a vivid demonstration that rainbows really exist as a continuum of spectral colors. Adjacent rainbows blend smoothly into each other, with no boundaries between red, orange, yellow, etc. This is clear evidence rainbows are not limited to 7 colors.
Dispersion of Light Causes Rainbows
The fundamental property of light responsible for rainbows is dispersion – different wavelengths or colors travelling at different speeds in a medium. This causes the various spectral components of sunlight to separate and spread out.
In a raindrop, violet light slows down more than red light as they enter the water. This difference in refractive indices for each wavelength leads to dispersion. Light disperses into the colorful spectrum we see projected in a rainbow.
Importantly, dispersion occurs equally and smoothly across all intermediate wavelengths. There are no gaps in the dispersion of sunlight into rainbow colors.
Rainbows are a Continuous EM Spectrum
Visible light that forms rainbows is part of the overall electromagnetic (EM) spectrum. This full spectrum ranges from long radio waves to short gamma rays. Visible light represents just one portion in the middle.
Within the visible light portion, wavelengths range continuously across an interval from about 380 to 750 nm. There are no separations between colors. Rainbows neatly demonstrate this continuity of the EM spectrum in the beauty of nature.
No Distinct Borders Between Rainbow Colors
Colors blend smoothly from one to another both within a rainbow and between multiple rainbows. This may be hard to discern to the naked eye, but physics proves there are no distinct borders or cut-offs between wavelengths.
Red does not suddenly end and orange begin. Yellow does not suddenly change to green. All intermediate colors and wavelengths exist to create a cohesive continuum.
Prisms Spread White Light into a Continuous Rainbow
Isaac Newton’s original experiments used glass prisms to demonstrate the dispersion of sunlight into a rainbow of colors. This same principle applies to all rainbows in nature, just using water droplets instead of glass.
In both cases, white light resolves into the visible color spectrum with no gaps or discontinuities. Passing through a dispersive medium, the colors blend continuously from long to short wavelengths.
Different Mediums Create the Same Continuous Rainbow
Not only do glass prisms and water droplets create rainbow dispersion, but any transparent medium can produce a rainbow. For example, droplets of dew or oil suspended in the air will generate rainbow reflections.
The specific colors and dispersion depend on the refractive indices of the material. But in all mediums, visible light disperses into a continuous spectrum with no cut-offs between colors.
Rainbows are White Light Minus Absorbed Colors
Another way to think about rainbows is that they display the visible colors that are not absorbed. As white light passes through a droplet, certain wavelengths are preferentially absorbed based on the molecular resonances of water.
The unabsorbed colors are then free to refract and reflect into the rainbow pattern. But since absorption varies smoothly across the spectrum, with no sharp cut-offs, the remaining rainbow light still forms a continuum.
Rainbow Formation Models the Electromagnetic Field
On a fundamental level, visible light is an oscillating electric and magnetic field propagating through space as electromagnetic waves. The range of possible wavelengths and frequencies of oscillation is unlimited.
When this continuous electromagnetic field interacts with matter, like a water droplet, the result is the continuous spectrum of rainbow colors. There are no discrete electromagnetic modes or separations.
Recreating Rainbows Requires a Continuous Light Source
Rainbows can be artificially generated by shining white light through a prism. Since the incident light source contains a continuous range of wavelengths, the output rainbow does as well.
Using a discontinuous or limited polychromatic light source could not recreate a proper rainbow. The continuum is an inherent result of the continuous electromagnetic emission from white light.
Rainbows are a Color Gradient
Another way to characterize a rainbow is as a color gradient. In computer graphics and imaging, gradients are created by smoothly blending from one color to another over a range.
This is analogous to how a rainbow transitions seamlessly between wavelengths, from long to short. There are no discrete color stops or boundaries in a computer-generated gradient, just like in a real rainbow.
Number of Rainbow Colors Depends on Observer’s Vision
The number of observable colors in a rainbow ultimately depends on the vision capabilities of the observer. The human eye can distinguish only about 100 hues. Other animals with different or broader color vision may see rainbows composed of more or less discrete colors.
But physically, rainbows comprise the full continuum of visible wavelengths, independent of biological detection. Rainbows contain infinitely many colors, disregarding human perception.
Conclusion
While rainbows are traditionally represented as 7 distinct color bands, modern physics and optics tells us rainbows actually contain an infinite and continuous spectrum of all colors. Rainbows serve as a beautiful natural demonstration of the continuous distribution of visible wavelengths in sunlight.
The 7 colors we learned in school represent rough categorical divisions, but not fundamental properties of rainbows. In reality, there are no discrete separations or boundaries between colors in a rainbow. The colors blend seamlessly from the longest red wavelengths to the shortest violet wavelengths.
So next time you see a rainbow, appreciate the gorgeous continuous distribution of colors it displays. Rainbows truly contain infinite colors beyond just the 7 you were taught!