The Opposition Effect

It was pretty obvious, an odd bright spot in the trees below that followed the helicopter. Having educated myself on quite a few optical phenomena I knew exactly what it was I was seeing, and made a point of taking a few photographs.

Opposition Effect
A bright spot in the Puna rainforest caused by the opposition effect
The mechanism for this bright spot is remarkably simple… No shadows.

Called the opposition surge, Seeliger effect or shadow hiding this simple optical phenomena occurs when looking at rough or irregular surfaces that are directly away from the light source, usually the Sun. On an irregular surface some parts will shadow other parts, resulting in an apparent darkening of the overall surface. When looking at that part of the surface directly away from the light source no shadows are seen, making that region appear brighter.

The effect is referred to as the opposition effect as it is the cause of a notable brightening of the Moon, asteroids, and the rings of Saturn that occurs at opposition, when the moon or planet is directly away from the Sun in our sky. The Moon grows as much as 40% brighter in the hours before and after full Moon. During opposition the rings of Saturn become brighter for several days as the ring particles no longer shadow one another. Asteroids have notably rough surfaces and can become much brighter near opposition.

Particles that can also retro-reflect light, such as dew droplets, sand grains, or glass beads can cause coherent back-scatter, this will also add to the effect on some surfaces. This is the mechanism that makes highway paints and road signs reflect your headlights so well, by means of small glass beads embedded in the paint. Coherent back-scatter, also sometimes called heiligenschein is at least part of the cause of the brightening of the full Moon, with shadow hiding probably responsible for the majority of the effect.

I really like the rain forest photo I took from the helicopter. It shows the opposition effect remarkably well and is likely all shadow hiding, without any coherent back scatter. You can see the shadows cast by each tree around the edges of the image and the lack of shadows in the center.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on the island of Hawaiʻi.

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