Road Bow

Last updated on September 16, 2023

Road Bow: A Phenomenon of Glass Beads

Have you ever looked down at the ground and seen a rainbow? It may seem impossible, but there is a phenomenon called a "road bow" that can create this mesmerizing sight. Unlike the traditional rainbow we see in the sky, a road bow is produced by small beads of glass on the surface of bricks or pavement. In this article, we will delve deeper into the fascinating world of road bows and explore the science behind them.

Glass is known for its ability to refract light, and it is this property that gives rise to the road bow. When sunlight passes through the glass beads, it undergoes a change in direction, causing it to separate into its component colors. This dispersion of light creates a primary rainbow with a radius of approximately 21 degrees. The Descartes diagram, which illustrates the paths of light rays, provides a visual representation of this phenomenon.

The width of the road bow is influenced by the sphericity and size of the glass beads. Beads with greater sphericity and larger size will produce a wider bow, while those with lesser sphericity and smaller size will result in a narrower bow. This variation in width adds to the uniqueness and beauty of each road bow.

In contrast to glass, water refracts light more weakly. As a result, when water droplets are present in the atmosphere, they create the familiar rainbow with a radius of approximately 42 degrees. However, it is important to note that road bows are not produced by water droplets. This distinction can be observed through careful analysis of the field of view of photographs capturing the road bow phenomenon.

When analyzing an image of a road bow, the field of view or focal length of the camera used plays a crucial role. Most mobile phone cameras have a field of view ranging from 50 to 70 degrees. Considering this range, if a water bow with an 84-degree diameter were to be captured, it would exceed the frame of the photograph. Additionally, environmental factors such as the absence of water in the air or on the ground further support the conclusion that the road bow is not formed by water droplets.

The captivating images of road bows captured by photographers like Abudulla Saheem showcase the beauty and rarity of this atmospheric phenomenon. These photographs serve as a testament to the power of nature and the wonders that can be observed even in seemingly ordinary settings like a dry car park.

In conclusion, road bows are a fascinating optical phenomenon that occur when sunlight interacts with small glass beads on the surface of bricks or pavement. The refraction of light through these beads creates a primary rainbow with a unique width determined by the characteristics of the glass beads. By understanding the science behind road bows, we can appreciate the intricate beauty of these rare phenomena and gain a deeper appreciation for the wonders of our natural world.

Road Bow pictured by Abudulla Saheem (photostream). ©Abudulla Saheem, shown with permission.

The broad bow centered on the antisolar point marked by the shadow of Abudulla's head is produced by small beads of glass at the surface of the bricks.

Glass refracts light strongly and produces a primary rainbow only 21 degrees or so in radius. The Descartes diagram at right shows the ray paths.

The bow width depends on the sphericity and size of the beads

In contrast to glass, water refracts more weakly and produces the huge and familiar rainbow of ~42° radius.

Large dewdrops on the ground produce a water bow. How do we know that this one was not produced by water drops?

The field of view or focal length of Abudulla's 'phone camera is unknown. Sony Ericsson stated "the information that you are requesting is something that we are unable to provide to end users". However, it seems that most mobile 'phones cameras have a field of view of 50-70°. A water bow of 84° diameter would more than fill the frame. Abudulla also notes that the image was taken "on a clear blue morning at a dry car park. There was no water in the air or on the ground. "

More glass bead bows (1,2).

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Reference Atmospheric Optics

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