Got the "Light Spot"

[wanttaja]

I've seen this effect for a number of years, but on an early-morning flight this week, managed to capture a good shot of it. See the bright spot in the center of the picture, right above the small grove of trees....

I think it's related to a "glory," except there isn't any prismatic effect.

It lies along the line of the aircraft's shadow projected towards the Earth. A bit of haze has muddied the sunbeam a bit, though, so the aircraft doesn't actually cast a shadow.

But the area around where the shadow would normally fall doesn't show any shadows, either, since the airplane is directly in line with the sun (if you look directly along a flashlight beam, you don't see any shadows cast by objects). Hence, the area on the ground looks bright because there are no shadows visible.

It's a bit weird when you watch it, because it remains at the same bearing and thus appears to be traveling along the ground on a parallel path....

Ron Wanttaja

 

[Cap'n Jack]

...
I think it's related to a "glory," except there isn't any prismatic effect.

....
Ron Wanttaja

It may be...

But could it possibly be a Poisson bright spot?

It goes by a couple of other names too...

In any case, thanks for sharing!
http://en.wikipedia.org/wiki/Arago_spot

 

[Skip Miller]

I think it's related to a "glory," except there isn't any prismatic effect.

Ron, I think it is a glory. It is just too bright to see the dimmer prismatic colors.

This opinion is totally fact free. Consume at your own risk.

-Skip

 

[wanttaja]

Ron, I think it is a glory. It is just too bright to see the dimmer prismatic colors.

I hope so. I'm to old to learn how to say, "Poisson".

Ron "Sounds fishy to me" Wanttaja

 

[Joe B]

It may be...

But could it possibly be a Poisson bright spot?

It goes by a couple of other names too...

In any case, thanks for sharing!
http://en.wikipedia.org/wiki/Arago_spot

Probably not. The Poisson/Arago bright spot depends on diffraction to brighten the lighting in the area where the shadow normally would be. That would mean the light as measured on the ground in the plane's "shadow" actually gets brighter than it was before or after the plane flew across the sun. I am not aware of anyone reporting that occurring anywhere, ever. It wouldn't be hard to test, though. Dunno if anyone ever thought to run the test.

I'm pretty sure, though, that it's not the glory based on the lack of prismatic effect. A glory depends on water droplets reflecting the light back at you, sort of like the tiny reflective glass beads they put in traffic signs. You see it frequently flying over clouds - but it usually has some sort of prismatic fringe. But no water drops means, by definition, you aren't looking at a glory.

I've seen the effect numerous times on dry ground and, like Ron, simply assumed it was due to the lack of shadow in that particular area. Dunno if anyone has done that test, either. Maybe we can call it the "Ron Effect"!

 

[Cap'n Jack]

Probably not. The Poisson/Arago bright spot depends on diffraction to brighten the lighting in the area where the shadow normally would be. That would mean the light as measured on the ground in the plane's "shadow" actually gets brighter than it was before or after the plane flew across the sun. I am not aware of anyone reporting that occurring anywhere, ever. It wouldn't be hard to test, though. Dunno if anyone ever thought to run the test.

I'm pretty sure, though, that it's not the glory based on the lack of prismatic effect. A glory depends on water droplets reflecting the light back at you, sort of like the tiny reflective glass beads they put in traffic signs. You see it frequently flying over clouds - but it usually has some sort of prismatic fringe. But no water drops means, by definition, you aren't looking at a glory.

I've seen the effect numerous times on dry ground and, like Ron, simply assumed it was due to the lack of shadow in that particular area. Dunno if anyone has done that test, either. Maybe we can call it the "Ron Effect"!

I tried to look for the bright spot during flying today- but I couldn't see it. I had to keep my eyes on more of a swivel today- the Ford was flying around, one of the airliners, and a helicopter flying around with the state fair, in addition to the usual traffic.

I agree it is something testable- maybe a group could work on it during a fly-in. If I could get out more I'd be happy to try it, or at least help with the experiment. We do see a bright spot where the shadow would be expected in the photograph, but I freely agree it may not be the Poisson effect. Remember that no one noticed the Poisson effect until Poisson predicted it as a consequence of Fresnel's diffraction theory (trying to refute the theory- who ever heard of a light spot in a shadow?)- Arago found the effect almost immediately

 

[gismo]

Probably not. The Poisson/Arago bright spot depends on diffraction to brighten the lighting in the area where the shadow normally would be. That would mean the light as measured on the ground in the plane's "shadow" actually gets brighter than it was before or after the plane flew across the sun. I am not aware of anyone reporting that occurring anywhere, ever. It wouldn't be hard to test, though. Dunno if anyone ever thought to run the test.

I see no reason why diffraction around the airplane wouldn't account for the bright spot.

 

[Joe B]

I see no reason why diffraction around the airplane wouldn't account for the bright spot.

Maybe it does. If it did, though, people on the ground would be able to notice things get brighter as an airplane flies across the sun. Airplanes have flown across the sun often enough you'd think someone would've noticed by now. Hence I favor a simpler theory. But maybe people just aren't that observant or the phenomenon happens too fast. So I think that the ubiquitous "somebody" ought to test it.

Edit: If diffraction were creating an actual bright spot we should be able to see the bright spots of other aircraft tracking around on the ground. Hmmm... something else to look for in the traffic pattern!

 

[gismo]

Maybe it does. If it did, though, people on the ground would be able to notice things get brighter as an airplane flies across the sun. Airplanes have flown across the sun often enough you'd think someone would've noticed by now. Hence I favor a simpler theory. But maybe people just aren't that observant or the phenomenon happens too fast. So I think that the ubiquitous "somebody" ought to test it.

Edit: If diffraction were creating an actual bright spot we should be able to see the bright spots of other aircraft tracking around on the ground. Hmmm... something else to look for in the traffic pattern!

Light can have direction and polarization as well as intensity and the effects can be counterintuitive when comparing light seen from different observation points. OTOH, I can't see how this would produce the effect described unless the ground was generating a specular reflection towards the airplane which would require some surfaces to be orthogonal to the line from the sun through the plane to the ground. A friend of mine has a book that explains the physics behind most unusual lighting effects including glorys and rainbows. I'll ask him if there's anything in it about this effect.

 

[Cap'n Jack]

Light can have direction and polarization as well as intensity and the effects can be counterintuitive when comparing light seen from different observation points. OTOH, I can't see how this would produce the effect described unless the ground was generating a specular reflection towards the airplane which would require some surfaces to be orthogonal to the line from the sun through the plane to the ground. A friend of mine has a book that explains the physics behind most unusual lighting effects including glorys and rainbows. I'll ask him if there's anything in it about this effect.

All you say is very true. There will be a distance component to it as well. The image on the left (looking like a shadow of a lollipop) was a disk (coaster taped to a thin wood rod). I moved the coaster further from the house- you may be able to see that the shadow appears more like an annulus rather than an evenly dark disk. I couldn't move further away since the shadow would be on the grass.