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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: enter image description here

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?

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A photo would be appreciated :) – PPG Jan 19 '14 at 15:12
its night here will have to wait – Dimensionless Jan 19 '14 at 15:15
up vote 5 down vote accepted

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.

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thanks a lot sir, even i saw many time the wire phenomenon and it explains everything thanks – Dimensionless Jan 19 '14 at 16:14

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.

enter image description here

3D plot of the shadow intensity:

enter image description here

The dip between two maximums looks like a bright area.

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What is shadow?

A shadow is an area where direct light from a light source cannot reach due to obstruction by an object. It occupies all of the space behind an opaque object with light in front of it.

enter image description here

I have tried the same experiment keeping my two fingers as the rods you showed, I didn't get such bright thing at the intersection point. I tried the same thing taking a semi-transparent pen and a non-transparent pen. At the intersection point, I observed the brightness. If you are asking the explanation for this case, the non-transparent pen produces a shadow, the semi-transparent pen has some light rays passing through it. Thus, the region appears little bright at the intersection point. You can see in the above picture stolen from wikipedia, where the shadow from transparent region appears little bright, and no such brightness is seen at the opaque part.

I didn't believe initially with olin lathrop answer. But when I saw the below picture, I really started to believe.
enter image description here
Actually there are white spaces in between the dark squares, but we are seeing dark spots in between (in your case you can exchange colors). It is the ghost image, an optical illusion. It is caused when our cone receptors lose sensitivity due to overstimulation. When I tried in home, my cone receptors were not over stimulated, so I didn't see bright spots at the intersection points.

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