A rainbow is a multicolored arc made by light shining through water droplets. The colors commonly associated with rainbows are red, orange, yellow, green, blue, indigo, and violet. However, one color that is notably absent from rainbows is white. This often raises the question – if a rainbow contains all the colors of the visible spectrum, why doesn’t it contain white?
There are a few key reasons why white is not present in rainbows:
1. Rainbows are caused by prismatic dispersion, which separates white light
Rainbows are optical phenomena caused by the refraction, reflection, and dispersion of sunlight through water droplets. When a ray of sunlight enters a raindrop, it bends (refracts) and reflects off the back inner surface of the droplet, then refracts again as it exits. This process separates the white light into its constituent spectral colors – the colors we see in the rainbow. Prismatic dispersion splits up the white light, so there is no white remaining to be seen.
2. Rainbows display a continuous spectrum of colors from red to violet
The sequence of colors in a rainbow arises from the fact that each color in the visible light spectrum has a slightly different wavelength. Red light has the longest wavelength while violet has the shortest. When white light is refracted by a water droplet, the component colors are bent by differing amounts according to their wavelength. Red light is bent the least while violet light is bent the most. This creates a continuous spectrum of colors across the arc of the rainbow from long wavelength red to short wavelength violet. There is no gap where white would be found.
3. White contains all colors of the visible spectrum mixed together
White light consists of all the visible wavelengths of light combined together. It contains red, orange, yellow, green, blue, indigo and violet. But in a rainbow, these component colors become separated. So there is no single point in a rainbow where all the colors converge that could produce white. The colors transition seamlessly from one to another in sequence by wavelength.
The Science Behind Rainbow Formation
To fully understand why white is absent from rainbows, it helps to look closer at how rainbows are formed scientifically:
1. Rainbows are produced by light reflection, refraction and dispersion in water droplets
Rainbows only occur when sunlight interacts with water droplets. As sunlight passes through a droplet, some light is reflected off the back of the droplet while the rest enters it. The light is refracted as it enters, reflected off the inside surface, then refracted again on the way out. This double refraction and reflection separates white light into colors.
2. Different colors in white light are bent by different angles
Refraction bends the light beam. The amount of bending depends on the wavelength. Longer red wavelengths are bent the least while shorter violet wavelengths are bent the most. This angular separation spreads out the component colors.
3. Dispersion causes light to spread out into a continuous spectrum
When refracted sunlight exits the raindrop, dispersion causes further spreading of the colors. This smears them into a continuous spectrum going from red on the top of the arc through orange, yellow, green, blue, indigo to violet on the bottom.
4. All colors transition seamlessly from one to the next
There are no gaps between the rainbow colors. The succession from red to violet is a smooth gradient with no possibility for white. The absence of white is just a consequence of the physical processes involved in splitting and dispersing the white sunlight.
Why Don’t We See White in a Rainbow?
Given the scientific explanation behind rainbow formation, we can summarize the key reasons there is no white visible:
White light gets split up into separate colors
Prismatic dispersion divides white into distinct colors ranging from longest red to shortest violet wavelengths. There is no white remaining after this separation.
There is no point where the colors re-converge
In a rainbow, the colors blend smoothly from one to the next without any point of overlap where they could recombine back into white.
White requires all visible wavelengths together
Since white contains all the rainbow colors combined, it can’t be seen in any single location but only when the spectrum is recombined.
Rainbows display the full color spectrum, not individual colors
Rainbows exhibit the entire visible spectrum, not selective individual colors. Since white exists outside this continuum, it is naturally excluded.
| Color | Wavelength (nm) |
|---|---|
| Red | 700-635 |
| Orange | 635-590 |
| Yellow | 590-560 |
| Green | 560-520 |
| Blue | 520-450 |
| Indigo | 450-440 |
| Violet | 440-390 |
When Can White Be Seen in Rainbows?
While uncommon, there are certain situations where white bands or spots can occasionally be seen in rainbows:
Fogbows
Fogbows are similar to rainbows but formed in fog instead of rain. The smaller fog droplets don’t disperse light as widely, allowing colors to blend back together into white regions.
Supernumerary rainbows
Extra bands of lighter pastel hues sometimes appear overlapping the main rainbow, caused by interference between light rays. These supernumerary bands may contain white.
Partial rainbows
If a rainbow doesn’t show the full sequence of colors, the limited spectrum can allow white light to persist in some areas.
Reflected rainbows
Rainbows reflected in water or other surfaces can show white spots where sunlight directly reflects off the surface underneath.
Raindrops larger than 1-2 mm
Large raindrops bend light less, reducing dispersion, so white light may not fully separate. But drops this big usually break apart.
So while rare, white can occasionally occur in rainbows under the right special circumstances where dispersion is limited. But in a typical rainbow, the complete spreading and separation of colors eliminates white.
How Do Rainbow Colors Reconverge into White Light?
Although white doesn’t occur at any point within the arc of a rainbow, the spectrum of colors can recombine to form white light again under certain conditions:
Reflection back through the raindrop
If dispersed rainbow colors reflect back off the inside of the drop, they can reconverge into white as they exit. But no light gets reflected backwards to the observer to reveal this white reconvergence.
Passing through a prism
A glass prism can serve as the reverse of a water droplet, recombining the separated colors back into white again. The dispersion is inverted.
Shining onto a white screen
Projecting a rainbow’s spectrum onto a white surface mixes the colors together again, revealing the white that was always there in the illuminating beam of sunlight.
So rainbow colors transition seamlessly from one to the next without white in between, but they can combine again to reconstitute white light under suitable conditions – just not within the rainbow itself.
Interesting Facts About Rainbows
Here are some fascinating facts about rainbows that provide additional insight into why white is absent:
– Rainbows are circular – we only see the arc
Rainbows form a complete circular band but we normally only see the section above the horizon. The rest is cut off by the ground.
– Rainbows have no set end points
The rainbow arc has no fixed beginning or end. It would form a continuous circle if we could see the whole thing.
– Rainbow colors are in spectral order from top to bottom
Red always appears on the outer top of the rainbow arc with violet on the inner bottom. The colors are arranged by wavelength.
– Anti-rainbows exist too
An “anti-rainbow” can form reflected off water droplets, showing reversed color order with red on bottom.
– Double rainbows can occur
Secondary and even tertiary rainbows can sometimes be seen outside the main one, with the color order inverted.
| Rainbow Facts |
|---|
| – Caused by light interacting with water droplets |
| – Colors range from red to violet by wavelength |
| – Forms a complete circular band |
| – No fixed beginning or endpoint |
| – Red always on top outer arc |
| – Violet always on bottom inner arc |
So in summary, white is not found in rainbows because prismatic dispersion separates and spreads out the colors smoothly, and there is no point where they can overlap to produce white again within the rainbow itself. This intriguing phenomenon arises from the underlying physics of light interacting with water droplets. The absence of white helps create the colorful splendor and beauty of one of nature’s most dazzling displays.
Conclusion
White is not present in rainbows because the process of light refraction, reflection and dispersion in raindrops separates white sunlight into the full continuum of visible colors. There is no gap or overlap in this spectrum where the colors recombine into white light. While rare optical effects can sometimes produce white in special types of rainbows, the typical rainbow arc displays a seamless sequence of hues ranging from red to violet without white. The blending, spreading and separation of colors as light interacts with water droplets explains this interesting omission of white from rainbows. So next time you see that colorful band of red, orange, yellow, green, blue and violet stretched across the sky, take a moment to appreciate the physical principles that produce this vivid demonstration of the spectrum – minus one color.
