Opposition Effect

September 11, 2005


Provided by: Roger Hopkins, Ruth's Waterfalls
Summary authors & editors: Roger Hopkins

The mid-morning Sun on a clear day in Comb Wash, Utah, shows a nice example of the "opposition effect." We took the photo from the edge of Comb Ridge, 800 feet (245 m) above Comb Wash, with the Sun directly behind us. The bright spot at the lower center is the "antisolar point" directly opposite the Sun. The trees, sagebrush, and grasses cover their own shadows at this point so that only directly reflected sunlight is observed. Toward the edges of the scene, shadows along with the reflected sunlight can be detected, and the image is darker. Go to the close-up photo (link below) to see this more clearly.

Unlike most forms of "atmospheric optics" resulting from refraction or diffraction; the opposition effect is simple reflection and does not depend on water, ice, dust or even an atmosphere. You can see this effect yourself as a halo-like brightness around the shadow of your head cast on a rough surface in bright sunlight, or even light from a full Moon. A grassy lawn or asphalt driveway seem to work particularly well.

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