Have you ever noticed a dark band in the sky between the primary and secondary rainbows? This peculiar optical effect is known as Alexander's Dark Band, named after Alexander of Aphrodisias, who first described it in 200 AD. In this article, we will delve into the details of this fascinating phenomenon and uncover the science behind it.
When sunlight interacts with raindrops in the atmosphere, it undergoes various reflections and refractions, giving rise to the beautiful spectacle of rainbows. The primary rainbow forms when light rays undergo a single reflection inside raindrops, while the secondary rainbow is formed by rays that are reflected twice. These reflections and refractions cause the sky inside and outside the rainbows to appear brighter.
However, between the primary and secondary rainbows, there exists a distinct dark band. This band appears darker than the surrounding sky, capturing the attention of keen observers. The reason behind this darkness lies in the behavior of light rays as they travel through raindrops along the lines of sight between the two bows.
In this region, the raindrops are unable to send light directly to our eyes. As a result, the sky appears darker because there is less light reaching our retinas. It is important to note that the dark band is not a physical entity but rather a visual perception caused by the absence of light from raindrops in that specific area.
The appearance of Alexander's Dark Band can vary depending on several factors such as the size and shape of the raindrops, the angle of sunlight, and the observer's position. These factors influence the path of light rays and determine the width and intensity of the dark band.
Interestingly, Alexander's Dark Band is not limited to rainbows alone. It can also be observed in other atmospheric optical phenomena such as glory, which occurs when sunlight interacts with water droplets or small ice crystals in clouds or fog. In these cases, the dark band appears between the glory and the surrounding bright rings.
To fully appreciate the beauty of Alexander's Dark Band, it is best to observe it from a vantage point where the primary and secondary rainbows are clearly visible. The dark band acts as a striking contrast, accentuating the vibrant colors of the rainbows and enhancing the overall visual spectacle.
In conclusion, Alexander's Dark Band is a captivating optical phenomenon that adds intrigue to the already enchanting world of atmospheric optics. As we gaze at rainbows and other optical displays in the sky, let us not forget to marvel at the subtleties and intricacies that make each experience unique. Next time you find yourself admiring a rainbow, take a moment to appreciate the enigmatic darkness that lies between its bows – Alexander's Dark Band.
Photo © Les Cowley
The sky between the primary and secondary bows is noticeably darker than elsewhere.
Alexander of Aphrodisias first described the effect in 200 AD and it now carries his name.
Light rays undergoing a single reflection in raindrops form the primary rainbow or brighten the sky inside it.
Rays reflected twice are deviated to form the secondary bow or brighten the sky outside.
Raindrops along lines of sight between the two bows cannot send light to your eye and so the sky is darker there.
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"Alexander's Dark Band". Atmospheric Optics. Accessed on November 30, 2023. https://atoptics.co.uk/blog/alexanders-dark-band/.
"Alexander's Dark Band". Atmospheric Optics, https://atoptics.co.uk/blog/alexanders-dark-band/. Accessed 30 November, 2023
Alexander's Dark Band. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/alexanders-dark-band/.