Great Wall Bow

Great Wall Rainbows imaged by Mark Vranicar (China blog) July 1st on a hike from the Jinshanling section of the Great Wall. ©Mark Vranicar, shown with permission.

The 'Great Wall' is a number of walls constructed from ~656 BC through to the 17th Century. This section was built in the Ming Dynasty (1368 - 1644) and is one of the best preserved. It includes 100 watchtowers as it strides over the mountains Northeast of Beijing.

Two rainbows created by a brief shower span the valley. The raindrops were relatively close but the rainbows, purely collections of light rays, look the same regardless of the shower's distance.

The background inside the bright inner primary bow is noticeably brighter. There, raindrops scatter sunlight to the eye and camera with a greater angular deflection than the minimum rainbow angle. The bow is a disk of light brighter at its edge. Classical ray theory actually predicts infinite intensity at the rainbow rim - a characteristic of caustics (1,2,3).

Between the primary and the fainter outer secondary is darkness. In that region the raindrops cannot direct any light towards the camera. This is Alexander's Dark Band.