A rainbow on the water surface of a swamp? It almost seems like the setting of a fairy tale, but instead it is a phenomenon that actually occurs. They are the so-called “rainbow swamps” or “iridescent swamps” with colored stripes ranging from blue, to green, to purple, up to golden yellow. The origin of this temporary phenomenon of iridescence it is completely natural: typically occurs at dawn or dusk in the autumn months when sunlight interacts with a subtle oily film produced by the decomposition of the organic material present above the swamp water. Conceptually it is the same phenomenon that is also observed in other situations iridescence on thin patinas as on the surface of a soap bubble or on a puddle in which there is oil spilled from a car.
Why some swamps are iridescent: the explanation of the phenomenon
It was once thought that rainbow swamps were the result of urban waste, contamination or pollution, but this is not the case: it is a natural phenomenon. The iridescence that may affect some bogs is due to the interaction between sunlight and the bog surface under certain conditions.
The leaves and tree bark (generally cypresses) present in the swamp fall and they rotenriching stagnant water with compounds derived from the decomposition and degradation of organic material. Responsible for decomposition are above all anaerobic bacteria of the Pseudomonas genus, normally present in these environments, which break down organic matter. These bacteria contribute to the formation of a oily film above the swamp water, because they produce surfactants which lower the surface tension of the water and allow the oil film to expand.
It is this oily film that causes the rainbow effect when hit by sunlight. A similar effect is observed when there is a little oil spilled from a car in a puddle. This phenomenon is called in technical jargon interference from thin films and occurs in the presence of a very thin film that separates two materials. In this case part of the sunlight (which contains all the colors of the rainbow) is reflected upwards by the oily film and part is instead refracted up to the lower edge of the film, to then be reflected upwards again. If the film is very thin these two rays are practically overlapped, therefore they interfere with each other. For some wavelengths of sunlight this interference it strengthens the corresponding colors, for others it weakens the corresponding colors. It is this geometric play that produces the colored bands that give certain swamps the effect of a rainbow on the water!
The effect is more intense as organic substances accumulate and the water becomes increasingly stagnant and swampy.
When and where are “rainbow swamps” observed?
The conditions due to the rainbow effect in swamps they are not found everywhere, and therefore this type of iridescence is not observable in all swamps in the world. There are places in particular where it is easier for this phenomenon to be encountered, and in some where the conditions are perfect and this event becomes a usual appointment.
Rainbow swamps are especially observable throughout the autumn season and to thebeginning of winterwhen the loaded with organic material lost from the trees and pouring into the stagnant water of the swamp. Furthermore, during these seasons theangle of refraction of light is perfect for the revelation of the phenomenon, especially especially at thesunrise ea end of the day. Furthermore, the perfect opportunity is to see them in a sunny day which it was preceded by a rainy day not too strong.
As for the places where this phenomenon is famous for being repeated over time, we have to fly to the USAparticularly in Carolina (in Congaree National Park), in Virginia (in the Great Dismal Swamp), in Georgia (in the Okefenokee swamp) and in Florida (in Everglades National Park).
