Rock Pool Corona

Rock Pool Corona

A corona around the sun's reflection in a rock pool seen by Aidan Forde on Deenish Island off the coast of Kerry, Ireland. The oval corona is generated by diffraction at the pool surface. Image ©Aidan Forde, shown with permission.

Imagine, as at left, just two points on an algal cell or a water droplet scattering the plane waves of incident sunlight. The two points become sources of outgoing spherical waves which eventually overlap. In some directions the crests of the two sets of waves are in phase to produce brightness. In other directions the waves are out of phase and they destructively interfere.

In reality, light is scattered from all points on the surface resulting in a bright central aureole fringed with red and surrounded by one or more coloured rings. The corona gets larger as the scatterers get smaller. When the scattering objects have a range of sizes the diffraction pattern is blurred and the rings are often not visible.

Aidan's corona is seen by reflection. The outgoing scattered waves are reflected by the pool surface.

There is another route - the sun's rays could be reflected upwards and then scattered by the algae.

Sometimes the waves from the two routes themselves interfere to produce extra fringes and colours on the water called Qu�telet fringes.

Scattering and diffraction of sunlight by small objects all of similar size on the pond surface must have been responsible. The corona's vertical elongation tells us that they appeared horizontally elongated from the direction of the sun's rays.

Algae or other single celled organisms were likely responsible. Many are globular and lie half submerged.

The diameters of the scatterers can be determined from the angular size of the corona - we might then identify the algae. Unfortunately, 'phone manufacturers are not photographers and omit the necessary data in image EXIF files and specifications!