Water's Coloured Shadows  Andrew Kirk captured these shadow colours cast through shallow and slightly turbid pond water.
©Andrew Kirk, shown with permission.

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Much is happening. The darkest shadow is purple.  Lighter ones at upper left are rusty red while those at lower right are greyish blue. The red shadows are further fringed with yellow at their tops and are purple edged opposite.   The pond bed is dimly lit in a reddish light.  Surface debris is in some places lit bright white and in others dimmer and bluer.

The key is the turbid water. Very clear water would show only a single shadow on the pond bed. Here there are three shadows: a 2D shadow on the pond surface, a 3D solid-volume shadow through the water and another 2D shadow on the pond bed. All three have a slight penumbrae where the sun is not totally obscured.

The pool surface reflects direct sunlight and blue skylight. The surface looks blue (rays from "A") where direct sunlight is shadowed. The debris there is dimmer and bluer too.

The solid shadow coursing through the water produces indirect colour effects. Dust motes and debris in the sunlit water scatter light. When they are very small, as here, they scatter blue light much more strongly than red. They are exactly analogous to the scattering by air molecules that produces a blue sky. As we get deeper and deeper in the pond the selective blue scattering reddens the remaining transmitted light.

Look along the ray direction "B" on the shadow side nearest to the eye. The blue scatterers at the upper pond levels are seen against the unlit shaft of the shadow. That shadow edge will appear blue - like the blue sky.

Look at ray direction "C" on the shadow's far side. Most of its upper path is unlit. Light comes from motes near the pond base scattering already reddened light and from the red illumed pond base itself. That side of the shadow appears red - Like a sunset landscape and sky.

The pool bed 2D shadow has its darkness relieved by a mixture of scattered blue and red light. It is then seen through scattered blue (equivalent to atmospheric airlight) and appears blue-purple.

Which effects predominate are altered by the viewing angle and the water turbidity.

Experiment with shadow colours and sunsets using a strong light shining into a bowl of water clouded with just a few drops of milk.