are the collective glints of millions of crystals which happen to have
the right orientation and angular position to direct their refracted light
into the your eye.
and crystals forming a 22º halo. Crystals
whose prism axes are roughly perpendicular
to the direction of sunlight refract its
rays through 22º
or more. Each crystal sparkles in the sky when
it is at this angular distance from the sun. The
collective sparkles make the halo. Millions
of other unlit crystals are not visible to you
but they might be helping to form someone else's
the sky contains "poorly aligned"
crystals pointing in nearly all directions, which ones glint and where?
How does so disorderly a host produce something so structured and ordered
as a 22º halo?
The key is still crystal orientation. Only those crystals with
their prism axes roughly perpendicular to the sun's rays allow
light to pass through two side faces.
These all deviate light by 22º to 50º to
form an illuminated disk with a 22º hole in its centre. The halo
is brightest near the hole.
When, as seen by the eye, they are also anywhere on a line 22º -
50º from the sun their light shines into the eye - they glint. But
all the lines of those angles form an annulus around the sun. The glints
'light up' the disk to form the halo.
And the other crystals that chance has neither placed nor aligned in positions
favourable for you? They are not seen but they could be helping to create
another person's halo.
A halo only exists if something is there to see it. It is the effect of
a collection of light rays travelling in particular directions and converging
on a receiving lens be it an eye or camera. Each person sees their own
halo. Someone standing only a short distance away sees another halo made
from the collective glints of another set of crystals.