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   Heiligenschein mechanism 

 
 
Dew drop near a leaf.


The droplet rests on small hairs which separate it from the leaf's surface.

The drop brings sunlight to a crude focus. When this is at the leaf surface, some light from the bright spot is scattered backwards through the drop to form the heiligenschein.
 

     The heiligenschein does not need very small droplets because it does not depend on diffraction. However, the drops must be small enough to hang on leaves in a special way.

Spherical dew drops act as lenses* and bring sunlight to a crude focus ~20% of their diameter beyond their rear facing surface. Some drops rest on the tips of small hairs and do not touch the leaf surface. When the drops and hairs have the right dimensions, sunlight is focussed in a bright spot on the leaf. Light from the spot scatters in all directions but some returns through the droplet along almost the same paths as it came. The net effect is that each drop 'backscatters' sunlight, returning it most brightly in directions towards the sun. Dew thus shines brightly at the antisolar point to form a heiligenschein.  But if the lawn or field is large enough check forty degrees away from the antisolar point for the same droplets can also form a speckled dewbow.

Why is the heiligenschein not green? It is said to look white because the individual points of light saturate the eye's colour receptors and to some extent cameras.

Nature is fond of lighting the antisolar point. The Glory arises from diffraction effects in small particles. Larger droplets resting on leaves give us the heiligenschein via geometric optics. Some minerals and crystals retroreflect. Forests, structured soils, the Moon and Mars appear to glow because of shadow hiding and coherent backscattering, the opposition effect. Even the dark night sky has its faint glow opposite where the sun rests, the gegenschein or counter glow made from light backscattered by interplanetary dust.


* Spheres make rather poor lenses. Rays passing close to the centre are more weakly focussed than those passing further away. This is spherical aberration.