The colorful pattern commonly known as a rainbow is scientifically called an optical spectrum. Rainbows are optical illusions that appear in the sky when sunlight interacts with water droplets in the atmosphere. The light is reflected, refracted and dispersed to create the multicolored arcs we see in the sky during or just after rainfall.
What causes rainbows?
Rainbows are caused by a combination of reflection, refraction and dispersion of sunlight by water droplets in the atmosphere. Here’s a closer look at how this happens:
- Reflection – When sunlight hits the back of a water droplet, some light is reflected off the back of the droplet. This reflected light is directed back towards the viewer, allowing the rainbow to be seen.
- Refraction – As sunlight enters the water droplet, it slows down and bends or refracts. The amount of refraction depends on the wavelength of light. Red light bends the least while violet light bends the most.
- Dispersion – As the refracted light exits the water droplet, the different wavelengths separate into the visible color spectrum. Shorter wavelengths like violet and blue bend more than longer wavelengths like orange and red.
The combination of these optical effects splits sunlight into the spectrum of colors we see in a rainbow. Different sized water droplets refract and disperse the light in slightly different ways, spreading the rainbow arc across the sky.
Rainbow color order
Rainbows always display colors in the same sequence, called the visible spectrum. The colors of the optical spectrum are, in order:
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet
Red appears on the outer edge of the rainbow arc since it is bent the least by water droplets. Violet appears on the inner edge of the arc since it bends the most. The different refraction indices for the wavelengths of light cause them to spread out into this color sequence.
Double rainbows
Sometimes a secondary, fainter rainbow arc is visible outside the primary rainbow. This is called a double rainbow. A double rainbow forms when sunlight is reflected twice within a water droplet before exiting it. The secondary arc displays the spectrum in reversed order with violet on the outer edge of the arc.
Double rainbows occur when there are enough water droplets in the atmosphere to produce the secondary reflection and refraction. The second rainbow is always fainter because more light escapes the water droplet after the second reflection.
Rainbow shape and size
Rainbows form circular arcs across the sky rather than complete circles or semi-circles. Several factors determine the shape and size of a rainbow:
- Viewing angle – Rainbows are optical illusions seen from a specific angle relative to the sun’s position in the sky. As you change your viewing angle, the rainbow shape will shift and change.
- Distance – The closer you are to the rainproducing the rainbow, the larger the rainbow will appear. Rainbows viewed from an airplane appear much smaller than those viewed on the ground.
- Sun height – The higher the sun is in the sky, the higher the rainbow arc will appear. With a very high sun, only the top of the arc may be visible.
- Raindrop size – Smaller raindrops tend to create more vivid rainbow colors and larger arcs than big raindrops.
Rainbows are nearly circular arcs spanning about 84° around the point opposite the sun. Typical rainbows are up to 15° tall and the largest rainbows extend up to about 18° in the sky. Very large rainbow arcs are possible when raindrops are small and the sun is very low in the sky.
Rainbow facts
Here are some additional interesting facts about rainbows:
- Rainbows are optical illusions, not physical objects. They cannot be approached or touched. You can only see a rainbow when your back is towards the sun.
- All rainbows are centered on the antisolar point, the point in the sky directly opposite the sun. A circular rainbow around the moon is sometimes seen around the antilunar point opposite the moon.
- Moonbows form just like rainbows, except sunlight reflecting off raindrops is replaced by moonlight. They are fainter white rainbows visible at night.
- Double rainbows can form with reversed color order, but triple rainbows are not possible because the third reflection within a raindrop does not exit at the right angle to be visible.
- Rainbows are commonly associated with leprechauns, unicorns, and pots of gold in myths and legends due to their magical and elusive appearance.
| Color | Wavelength (nm) | Frequency (THz) |
|---|---|---|
| Red | 620-750 | 400-484 |
| Orange | 590-620 | 484-508 |
| Yellow | 570-590 | 508-526 |
| Green | 495-570 | 526-606 |
| Blue | 450-495 | 606-668 |
| Indigo | 420-450 | 668-714 |
| Violet | 380-420 | 714-789 |
This table shows the wavelength and frequency ranges for each color of the visible light spectrum.
How are rainbow colors produced?
The different rainbow band colors are produced based on the wavelength and frequency of each color of visible light:
- Red – The long wavelengths of red light near the infrared range are bent the least by water droplets, causing red to appear on the outer edge of the rainbow.
- Orange – Slightly shorter orange wavelengths are bent a bit more than red light, positioning orange next to red in the spectrum.
- Yellow – Yellow light, with a medium wavelength, takes on an intermediate position between the reds and greens of the rainbow.
- Green – The medium wavelengths of green light are refracted more than red and orange but less than blue and violet light.
- Blue – Shorter blue wavelengths are positioned farther along the spectrum since they are bent more than preceding colors.
- Indigo – With slightly shorter wavelengths than blue light, indigo sits next to violet at the innermost arc of the rainbow.
- Violet – The shortest violet wavelengths near the ultraviolet range are bent the most, displaying violet along the inner edge of the rainbow.
Light waves are spread based on their refraction through raindrops, sorting them by wavelength and frequency into the sequence of rainbow colors.
Why are rainbows colorful?
Rainbows display the color spectrum because sunlight is composed of a broad spectrum of wavelengths from long infrared to short ultraviolet. When this full spectrum of sunlight enters raindrops, the different wavelengths are refracted at different angles, causing them to disperse and separate. Shorter wavelengths are more strongly refracted than longer wavelengths, creating the visible color spectrum we see in rainbows.
If sunlight was a single wavelength, rainbows would not be multicolored. The wide range of wavelengths in sunlight, coupled with the wavelength-dependent refraction by raindrops, makes rainbows split into the colors of the visible light spectrum.
Where can rainbows be seen?
Rainbows can be seen anywhere that the right conditions occur together:
- Sunlight is able to shine on falling rain
- Water droplets or mist are present in the air
- The sun is fairly low in the sky
- Your back is to the sun as you look up at around a 40-42 degree angle
As long as these criteria are met, rainbows can occur anywhere from open country to city streets to mountain wilderness. Two key factors are sunshine interacting with rain and the proper viewing angle relative to the sun’s position.
When are rainbows visible?
Rainbows are most commonly seen in the early morning around sunrise or late afternoon around sunset because this is when the sun is low in the sky. The low sun angle allows your line of sight to align with the optical path for spotting a rainbow. Rainbows can form at any time of day, but are most frequently observed before noon or later afternoon.
Rainbows are also more likely when rain is moving away and skies are clearing rather than during heavy downpours. Light rain or mist provides ideal conditions for rainbows since heavy rain often obscures sunlight and reduces visibility.
Conclusion
In summary, rainbows are multi-colored optical illusions that occur when sunlight interacts with water droplets in the atmosphere. The light is reflected, refracted, and dispersed into the visible color spectrum based on wavelength. Rainbow colors always appear in the sequence red, orange, yellow, green, blue, indigo, violet, with red on the outer band and violet on the inner band.
Double rainbows can occur when a secondary reflection forms, creating a fainter inverted rainbow outside the primary bow. Rainbows are circular arcs centered on the antisolar point opposite the sun’s position. Their shape and size varies based on viewing angle, distance, sun height, and raindrop size.
Understanding the optics involved helps explain how these colorful meteorological phenomena brighten our skies after rainfall. Rainbows have inspired awe, stories, art, and science across human cultures for centuries.
