Sampson shortly before landing at Whitehorse, Yukon in June
'08. Image ©Russell Sampson,
shown with permission.
"It was about 30 minutes
before sunset and there was a great opposition effect in the distant
pine trees. It
was a rather extreme contrast situation with low light levels. Nonetheless
I was impressed with the brightness of the effect."
The opposition effect is a bright patch at the antisolar point (ASP)
directly opposite the sun. In this case part of the effect is caused
by shadow hiding. The shadows of the trees directly opposite the sun
are hidden by the trees themselves. Away from the ASP the
shadows become increasingly visible (unhidden) and, counterintuitively,
they appear closer to the ASP than the shadow casters. The lack of shadows
at the ASP makes it appear brighter than the surrounding area. It also
appears more yellow because it lacks bluish sky lit shadows.
That's not the whole story, retro-reflection from minerals sometimes
brightens the ASP and there is also an interference effect - coherent