Skypools on the surface of Walker Pond in Sedgwick, Maine. Imaged by Larry Peterson.  
©Larry Peterson, shown with permission.

Atmospheric
Optics

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The ever changing swirls and ovals are reflections of blue sky, white clouds, a nearby shaded tree and the distant landscape.

But that is too bald a description for skypools are strangely complex. They show sky and land colours but not sky or landscape pictures. They are instead ovals of alternating colour. Some ovals are mostly landscape hues, others are of the sky.

The complexity arises from the shape of the water surface. Ordinary mirrors are flat, convex or concave. They produce simple and single images. They have a fixed curvature.

In contrast, wavy water even at a single 'frozen' instant changes in its curvature from point to point along its surface. Its troughs are certainly concave at their deepest but the curvature decreases towards the crests passing through an inflection point and then becoming convex.  And that is only in one dimension, the curvature alters differently in others.

At right is an accurately computed ray diagram for a particular viewing position and (cylindrical) waves. The blue coloured rays come downwards from directions in the sky. The yellow rays are their reflections upwards into the eye.

Letís follow what happens as you look towards position Ďaí on a wave and then slowly look further outwards across the water surface.  Moving from a to b we see reflections from points in the sky getting closer to the horizon. This is an inverted image.

From b to c the reflected rays instead come progressively from higher and higher points in the sky. This is a region where the reflection is upright.

From c to d and onwards the reflection again comes from lower and lower points in the sky as we look further along the wave. An inverted image.

In the next wave trough the process repeats itself to give more upright and upside-down sky views.

At e a further complication starts!   Some rays are reflected twice by the water surface until they reach the eye. This adds yet more images. For clarity the diagram avoids showing these twice reflected rays.

Real water has undulations of several wavelengths and extending in other directions resulting in mainly oval bands of different reflections.

Larry Peterson's location had dark trees where he was standing and some of his 'skypools' image them to become 'landpools'. Elsewhere we see roughly concentric ovals of reflected cloud and reflected blue sky.

The instant eye of a camera best sees skypools. When they are evident act quickly and take many images for a small change in the wind and they are gone.

Skypool light comes from many directions.   When the incident light is parallel - from the sun, moon or distant lights - the water instead shows glints and glitter paths.   Sometimes moon reflections appear to the eye as skypool like rings but a camera reveals them instead as darting sharp points of light for then we are seeing the effects of sharp caustics sweeping across the eye rather than the gentle rays of skypools.