Dew drop near a leaf.
The droplet rests on small hairs which separate it from the leaf's
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
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.
make rather poor lenses. Rays passing close to the centre are
more weakly focussed than those passing further away. This is