Dewdrop Optics

November 12, 2018

Dew_on_banana_leaf_29Sep2018_for_EPOD_s (3)

Photographer: John W. Ehman
Summary Author: John W. Ehman 

These dewdrops in the bright early morning sunshine show two different yet related ways how they act like little lenses. First, they magnify the area beneath them, acting like a planar-convex lens, as the droplets are somewhat flattened in contact with the leaf -- here they're on a banana leaf. However, because banana leaves are quite waxy, they offer a relatively low adhesion factor in relation to the water droplets' internal cohesive properties, with the result that the droplets prominently bead.

By these beads standing fairly tall, and the sunlight coming from a sharp/side angle, they act much like bi-convex (spherical) lenses, with characteristically very short focal lengths. Notice how light rays are focused into bright points on one side for many of the droplets. This effect is highlighted by the points' occurrence in the shadows of the droplets themselves, with those shadows being created by the diversion of light on the opposite side of the droplets. So, the water is letting in some light and focusing it on one side, while the surface of the droplets (caused by water's cohesive properties) is nevertheless on the other side bouncing some light back, with the effect being that the droplets cast shadows that serve as backdrops for the short focal points. Photo taken on September 29, 2018, near Upper Darby, Pennsylvania.

Photo Details: Olympus OM-D EM-5 camera; Olympus 75 mm f/1.8 lens; f/10; 1/200 second exposure; Focal Length (35mm): 150; Software: Adobe Photoshop Elements 12.0 Macintosh. 

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