Primary Rainbow

Primary Rainbow: Exploring the Phenomenon

Rainbows have long captivated our imagination with their vibrant colors and ethereal beauty. These captivating optical phenomena occur when sunlight interacts with raindrops in the atmosphere, creating a mesmerizing display of colors. While most of us are familiar with the primary rainbow, there is much more to this natural spectacle than meets the eye.

The Anatomy of a Rainbow

Contrary to popular belief, a rainbow is not simply a set of colored rings. Instead, it is a disk of light that forms a circular shape in the sky. The primary rainbow is the most prominent and commonly observed type. Its outermost color is red, followed by orange, yellow, green, and blue. The colors blend seamlessly into one another, creating a stunning arc of hues. Occasionally, when raindrops are small, faint supernumerary arcs of electric greens, pinks, and purples can be seen just inside the main bow, adding an extra touch of enchantment to the spectacle.

The Brightness Within

One intriguing aspect of rainbows is the stark contrast between the brightness inside the bow and the surrounding sky. The reason for this lies in the nature of rainbows as disks of light rather than sets of colored rings. As sunlight passes through raindrops and is refracted, it is also internally reflected within the droplets. This internal reflection directs light towards the center of the rainbow, causing it to appear brighter compared to its surroundings.

The Elusive Nature of Rainbows

While rainbows may seem like a common occurrence, they are actually quite rare. In any given location, the number of bright rainbows that can be observed in a year is typically fewer than ten. Factors such as the availability of sunshine and falling rain play a crucial role in their formation. Furthermore, rainbows are always opposite the sun in the sky, with their centers located below the horizon at the antisolar point. Therefore, the lower the sun is in the sky, the higher the rainbow appears.

Optimal Conditions for Rainbow Spotting

If you're eager to catch a glimpse of a rainbow, timing is everything. The best opportunities to witness this awe-inspiring phenomenon are during the early morning or late afternoon when the sun is not too high in the sky. This positioning ensures that the rainbow is opposite the sun, allowing for its full glory to be revealed. So, keep an eye out for rain showers and position yourself strategically for the best chance of witnessing this natural marvel.

The Secondary Rainbow

Beyond the primary rainbow lies a secondary bow, which is fainter in appearance. This secondary bow is formed by a different process known as double reflection. The light undergoes an additional reflection within the raindrop before exiting, resulting in an inverted color sequence compared to the primary rainbow. The secondary rainbow appears at a larger angle from the antisolar point and is often dimmer due to greater light dispersion within the droplets.

Rainbows Around the World

Rainbows can be observed in various locations around the world, each offering its own unique backdrop for this captivating display of light and color. From vibrant cityscapes to serene countryside settings, rainbows add a touch of magic to any landscape they grace. Whether you find yourself in rainy England or under the brilliant skies of Austria, the allure of rainbows transcends borders and cultures.

Capturing the Beauty

For those fortunate enough to witness a rainbow, capturing its beauty through photography can be a rewarding experience. The vibrant colors and mesmerizing arc make for stunning visual compositions. Experimenting with different angles and perspectives can enhance the overall impact of your photographs. So, don't forget to bring your camera and seize the opportunity to immortalize this fleeting wonder of nature.

Appreciating Nature's Wonders

Rainbows are not only a visual delight but also serve as a reminder of the fascinating scientific principles at play in our natural world. From the refraction and dispersion of light to the intricate interplay between raindrops and sunlight, rainbows offer us a glimpse into the magic of optics. Taking a moment to appreciate these natural wonders can inspire a sense of awe and wonder, reminding us of the beauty that surrounds us.


The primary rainbow, with its vibrant colors and luminous disk, is a captivating phenomenon that continues to mesmerize people around the world. Understanding the science behind rainbows and the optimal conditions for their formation allows us to appreciate them even more. So, the next time you find yourself beneath a rain shower with the sun peering through the clouds, take a moment to look up and marvel at the breathtaking spectacle of a primary rainbow.

Bright primary rainbow over Schlägl, Austria.

The sky is brighter inside the bow because rainbows are disks of light rather than sets of coloured rings.

Outside the primary there is a fainter secondary bow.

Imaged by Karl Kaiser (site) on August 28, 2002.

©2002 Karl Kaiser, shown with permission.

To see a rainbow we need sunshine and falling rain. Rainbows are rarer than might be thought. In any one place in rainy England there are fewer than ten bright ones in a year. Halos occur much more frequently.

Early morning and late afternoon are the best times to see them because the sun must not be too high. Rainbows are always opposite the sun and their centres are below the horizon at the the antisolar point. The lower the sun the higher is the bow.

Red is always outermost in the primary bow with orange, yellow, green and blue within. Occasionally, when the raindrops are small, fainter supernumerary arcs of electric greens, pinks and purples lie just inside the main bow.

A rainbow is not just a set of coloured rings. The sky inside is bright because raindrops direct light there too. The primary bow is a shining disk brightening very strongly towards its rim.

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Reference Atmospheric Optics

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