Interesting optical effect

Maule Driver wrote:

Actually I see it on the ground but it is diffuse and unfocused - like a
very strong headlight shining on a very distant area. You just get a
diffuse bright spot.

I think I get Peter's concept and understand it but can't quite get it to
'intuit'. But makes some sense. Can't quite understand why the area on the
ground where sunlight would reflect back at 180deg (or normal to the average
surface) would be so much brighter than points where the sunlight would
reflect back at say 45 deg to the avg surface. Doesn't make sense when I
think of a forest of trees, but it is quite clear when same light refects of
a truck on a highway. So I kind of get it. Thanks Peter and Robert.

Peter didn't really explain it. Except for specially-designed surfaces that
incorporate corner reflectors (like taillights, or bicycle reflectors), a
randomly-oriented surface doesn't preferentially reflect light back in the
direction of it's source. However, the effect has a very simple explanation:

The bright spot is simply the result of the absence of SHADOWS cast by objects
on the ground. On a sunny day, EVERY object on the ground - rocks, blades of
grass, trees, people - casts a SHADOW. When one looks at the ground from a
considerable distance, each tiny object and it's *shadow* are all averaged
together to contribute to the total brightness of the surface. However, when
one looks at the ground from the direction of the Sun, every shadow DISAPPEARS
behind the object that casts it! As a result, in that ONE direction there are
NO SHADOWS to dilute the intensity of the light reflected back to the observer;
the result is a *bright spot* in which the apparent reflectance is very nearly
DOUBLE that in any other direction.

The effect is not noticed if you are standing on the ground (or very near the
ground in an airplane) because your OWN shadow blocks the sunlight in that
direction. When the airplane climbs to a considerable height (such that it
appears much smaller than the Sun as seen from the ground) the shadow of the
airplane ceases to exist and no longer blocks the light in the "anti-sun"
direction, allowing the effect to be seen.