Think You Know Rainbows? Look Again [Slide Show]
Physicists are starting to explain how rainbows can be completely red, white, nearly flat or full-circle
![Think You Know Rainbows? Look Again [Slide Show]](https://static.scientificamerican.com/sciam/cache/file/BF7661B1-3434-41E0-BDF92A27290BACE3.jpg?w=590&h=393)
Think You Know Rainbows? Look Again [Slide Show]
- CLASSIC RAINBOWA single-arc rainbow appears when sunlight reflects just once inside each of the many airborne droplets left by a rainstorm before it exits. A second, dimmer rainbow with inverted colors (top left) can appear above this primary rainbow if some of the incoming light is reflected twice before it escapes. Light exits the drops after the first and second reflections at slightly different angles, offsetting the two rainbows in the sky. The secondary rainbows are always there but, due to their faded colors, are not always visible.Photo: Courtesy of Klafubra on Wikimedia Commons, CC BY-SA 3.0
- FOGBOW Sunlight shining through fog gives rise to a white bow. Fog drops are at least an order of size smaller than raindrops. Rather than splitting the colors into well-defined bands, the small drops create much broader bows that overlap and merge into a ghostlike arc. Only the lens-focusing effect of the drop's surface remains distinct, maintaining the high-intensity white bow. Photo: Courtesy of Christopher Michel on Wikimedia Commons, CC BY 2.0
- BLOOD RED BOWWhen the sun lies just on the horizon, its light has to travel through more of Earth's atmosphere than at any other time of the day before it reaches the falling drops from a rainstorm. Air molecules scatter blue rays the most (which is why the sky is blue) but at sunrise and sunset, the light path is so long that green, yellow and even orange can also be scattered significantly; only red makes it through unscathed, creating a monochromatic red arc.Photo: Courtesy of Jason Campbell on Wikimedia Commons, CC BY-SA 3.0
- SUPERNUMERARY ARCSAdditional bands appear below the main arc of a rainbow when the rain shower contains drops that are all about the same size. These bands arise from interference of the light as it exits the drops. Due to the curvature of a drop's surface, not all of the light rays travel along the same path as they traverse the drop; some rays travel farther than others. When the rays exit the droplets, the light wave's crests and troughs no longer may all line up. Additional bands appear when the waves reinforce one another and the sky is dark where they cancel out. If the raindrops are not very similarly sized, these bands disappear.Photo: Courtesy of Eric Rolph on Wikimedia Commons, CC BY-SA 2.5
- DARK BANDThe band of sky between two bows is always darker than the region inside the first and outside the second. This is due to the curvature of their droplets. Almost none of the light reflected by drops between the two bows reaches the observer, so this part of the sky appears darker.Photo: Courtesy of Mike Quinn on Wikimedia Commons, CC BY-SA 2.0
