Sky Pools & Alphabet Soup ~ Skypools are the ever moving rings and swirls of light on wavy water. They are the not so simple reflections of the sky and surroundings. These ones in a water trough surrounding a statue were imaged by Frank Le Blancq in Broad Street, St. Helier, Jersey.     ©Frank Le Blancq, shown with permission.

The scene is at left. We see fragments of the blue, white and red shop awning in the reflections. Some reflections are wrapped tight into closed loops. At lower left in the main image is something more telling, fragments of words. They are the reflections of the "BROAD STREET" sign just visible on the wall of the white building between the two windows. Its reflections form an 'alphabet soup'. Some are the right way up, others inverted, others twisted. The sky reflections in ordinary skypools are hard to unscramble. The Broad Street alphabet soup shows that the reflections are not simple and straightforward.

'BROAD STREET is inverted in the upper part of the skypool loop. A fragment EET is the right way up at lower right. Part of another STREET reflection is at lower left. Skypools are made up of several individual reflected images, some inverted, some not.

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed
The complexity of skypools comes from the involved water surface contours. Ordinary mirrors are flat, convex or concave. They produce simple and single images. They have a fixed curvature.

Wavy water is not so simple. Even at a single 'frozen' instant it changes 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 two-dimensional sinusoidal waves. The blue coloured rays come downwards from the surroundings including the "Broad Street" sign. The yellow rays are their reflections back upwards into the eye.

Letís follow what happens as you look towards position Ď
aí 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.

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.

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 before 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 differing in other directions resulting in 'concentric' oval bands of reflections.

Frank Le Blancq's images with identifiable reflections neatly demonstrate the subtlety of water surface patterns.