Q:

Why does the point of intersection of two shadows look brighter?

When I sit in my garden during the day I always keep looking at the two intersecting shadows of two rods.  It looks something like this:

What I find weird about it is that at the point of intersection of the shadows, instead of becoming darker, they become bright. Can anybody explain why this happens?

A:

There is not only the optical illusion. Rods are thin so shadow is smooth (the Sun isn't a point source). The smoothness produces a dip of the shadow intensity at the rods intersection.

3D plot of the shadow intensity:

The dip between two maximums looks like a bright area.

A:

What you are seeing is a optical illusion.  Think of the shadow from each rod as darkening the ground by one unit.  On average, you have two units of darkening from the two shadows combined.  However, where the shadows cross there is only one unit of darkening.  Your visual system "sees" this local relative brightening as bright spot.
I usually notice this effect when looking at electric wires between utility poles against the sky.  Each wire looks black, but there appears to be a bright spot where they cross from my vantage point.
The human visual system is heavily driven by relative differences as apposed to absolute brightness.  Usually that works really well, but various optical illusions result in corner cases.

