A Waterfall's Optical Engine
Skógafoss is formed by geological and glacial mechanics. The waterfall is a sheer, 60-meter-high curtain of water, 25 meters wide, that drops from a former sea cliff. Thousands of years ago, Iceland's coastline was located here, but post-glacial isostatic rebound—the rising of land after the immense weight of glaciers was removed—shifted the coast 5 kilometers south, leaving the cliff behind. The Skógá River, which feeds the waterfall, originates from the meltwater of the Eyjafjallajökull and Mýrdalsjökull glaciers, ensuring a powerful and consistent flow year-round.
This immense and constant volume of water creates the waterfall's unique atmospheric conditions. As the water plunges over the hard basalt cliff edge, it atomizes into a vast, dense cloud of mist. It is this spray that makes Skógafoss produces rainbows. On any sunny day, the conditions are almost always right for a single or double rainbow to form within the mist.
The Physics of Frequent Rainbows
A rainbow is an optical phenomenon caused by the refraction and internal reflection of sunlight within water droplets. For a bright rainbow to appear, two main conditions must be met: a source of sunlight and a suspension of water droplets of a relatively uniform size. Skógafoss excels at producing the latter. The energy of the 60-meter drop creates a consistent distribution of water droplet sizes ideal for separating sunlight into its constituent colors.
Light entering a droplet bends (refracts), reflects off the inner back surface of the droplet, and bends again as it exits. The angle of exit for this process is approximately 42 degrees, which determines the position of the primary rainbow. The intense spray at Skógafoss often produces a secondary, fainter rainbow as well. This double rainbow is the result of a second internal reflection within the water droplets, causing light to exit at an angle of around 50-53 degrees and reversing the order of the colors. The sheer quantity of mist particles at Skógafoss makes these optical phenomena exceptionally common.