Caustic Reflections and More

September 25, 2017

Bridge - Gail 1 (1)

Photographer: Gail Garner 
Summary Authors: Dave LynchGail Garner 

The photo above shows several interesting reflections. It was taken at the Cass Street Bridge on the Boardman River in Traverse City, Michigan. The brownish light reflected from the water in the middle foreground is actually a third order reflection, which in general is not unusual but also not commonly recognized by observers. Sunlight reflects off the water in part of the scene that we cannot see and on to the underside of the bridge (first order reflection). This reflection produces caustics that can be seen as bright areas on the underside of the bridge. This light then reflects (second order reflection) off the underside of the bridge and onto the water below. After reflecting off the shaded water’s surface (third order reflection), the dark brown colored surface, it reaches the observer’s eye, or in this case the camera. That the Sun is low in the sky is evident from several aspects of the landscape illumination.

The bright brownish color on the underside of the bridge is probably that color for two reasons: (1) the Sun is low in the sky and probably somewhat yellowish (the spectra for yellow and bright brown or for brown and dark yellow is basically the same); (2) the river here is shallow so sunlight penetrates the water, reflects off the brown sediment in the riverbed, then reemerges and illuminates the underside of the bridge. Photo taken at 9:45 a.m. August 21, 2017. 

Photo Details: Camera Maker: Apple; Camera Model: iPhone SE; Lens: iPhone SE back camera 4.15mm f/2.2; Focal Length: 4.15mm (35mm equivalent: 49mm); Digital Zoom: 1.669x; Aperture: ƒ/2.2; Exposure Time: 0.0071 s (1/140); ISO equiv: 25; Software: Adobe Photoshop CC 2017 (Windows).