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Author Topic: Why does an airplanes shadow become a bright spot?  (Read 4988 times)

Offline Eric A. Taylor

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This is something I've wondered about for years and years. When I was growing up my dad worked part time as a flight instructor. We had a part interest in a small plane, a Cessna 177RG, kind of the Corvette of the Cessna fleet. As we'd take off I liked to watch our shadow on the ground. As we climbed higher and higher the shadow would become more and more blurry as it got smaller and smaller, then just before it vanished it would become a bright spot. At a 1 or 2 thousand feet up I'd look down and see a bright spot on the ground zipping along with us. I know that a small single engine, 4 seat plane has far too little mass to cause gravity lensing, but what causes this?
Eric in Portland


 

Offline Soul Surfer

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Why does an airplanes shadow become a bright spot?
« Reply #1 on: 21/12/2009 09:33:01 »
This is possibly caused by the airflow over the aircraft.  There are high pressure spots and low pressure spots associated with the process of living the lift that keeps the aircraft in the air the low pressure spots are above the wings and therefore in hidden but some of the high pressure will trail behind the aircraft.  high pressure will cause an increase in the density of the air locally and this can act as a positive lens.  this provess coud work over any terrasin

There is also another bright spot process associated with cloud formation  this is called "the Brocken Spectre"  and can be seen around the shadow of your head in sunshine and light mist on mountains.  It is caused by reflection from water droplets.  I have seen this on my shadow on dewy grass in the early morning so if you are flying across fields with dewy grass it could be due to direct reflection from water droplets on the ground at the point where your shadow should be.  this would oly work in terrain and conditions where small water droplets are to be expected.
 

Offline RD

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Why does an airplanes shadow become a bright spot?
« Reply #2 on: 21/12/2009 10:30:48 »
Diffraction can cause a bright spot in the centre of a shadow ... http://en.wikipedia.org/wiki/Arago_spot


http://www.sciencephoto.com

« Last Edit: 21/12/2009 10:34:59 by RD »
 

Offline Eric A. Taylor

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Why does an airplanes shadow become a bright spot?
« Reply #3 on: 22/12/2009 02:32:43 »
This is possibly caused by the airflow over the aircraft.  There are high pressure spots and low pressure spots associated with the process of living the lift that keeps the aircraft in the air the low pressure spots are above the wings and therefore in hidden but some of the high pressure will trail behind the aircraft.  high pressure will cause an increase in the density of the air locally and this can act as a positive lens.  this provess coud work over any terrasin

There is also another bright spot process associated with cloud formation  this is called "the Brocken Spectre"  and can be seen around the shadow of your head in sunshine and light mist on mountains.  It is caused by reflection from water droplets.  I have seen this on my shadow on dewy grass in the early morning so if you are flying across fields with dewy grass it could be due to direct reflection from water droplets on the ground at the point where your shadow should be.  this would oly work in terrain and conditions where small water droplets are to be expected.

Apparently you didn't read my post very carefully. My father was an IP so I think I understand how airplanes fly a little better than you do. The high pressure/low pressure explination is a simplified explanation of lift.

What's really happening is the shape of the wing pushes air downward. For every kilogram of air pushed down you get a kilogram of lift. Air is a lot more heavy than you might think. A typical room in your house can contain several tons of air, so a light plane that weighs around 2,000 pounds doesn't need to push down very many cubic feet of air to get enough lift to fly. I don't think the density diffrence would be enogh to notice from 2000 feet up.

Diffraction can cause a bright spot in the centre of a shadow ... http://en.wikipedia.org/wiki/Arago_spot


http://www.sciencephoto.com



Sorry, don't think that's right. A Cessna 177rg is not round and the sun is not a point source of light.

"In optics, an Arago spot is a bright point which appears at the center of the shadow of a circular object in light from a point source."
 

Offline AllenG

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Why does an airplanes shadow become a bright spot?
« Reply #4 on: 22/12/2009 04:09:06 »
Diffraction can cause a bright spot in the centre of a shadow ... http://en.wikipedia.org/wiki/Arago_spot


http://www.sciencephoto.com



Sorry, don't think that's right. A Cessna 177rg is not round and the sun is not a point source of light.

"In optics, an Arago spot is a bright point which appears at the center of the shadow of a circular object in light from a point source."


I think that is on the right tract actually.  I don't know that one could see well enough to resolve the shape of a point source of light from altitude, but the contrast difference may be noticeable and your brain interprets this as a bright spot.
 

Offline Eric A. Taylor

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Why does an airplanes shadow become a bright spot?
« Reply #5 on: 22/12/2009 06:16:52 »
Diffraction can cause a bright spot in the centre of a shadow ... http://en.wikipedia.org/wiki/Arago_spot


http://www.sciencephoto.com



Sorry, don't think that's right. A Cessna 177rg is not round and the sun is not a point source of light.

"In optics, an Arago spot is a bright point which appears at the center of the shadow of a circular object in light from a point source."


I think that is on the right tract actually.  I don't know that one could see well enough to resolve the shape of a point source of light from altitude, but the contrast difference may be noticeable and your brain interprets this as a bright spot.

I'm not sure that's right. From the altitude of 2000 the sun looks the same as it does on the ground. In fact the sun is so far away you have to get several million miles closer for it to look any bigger. The disk of the sun, as seen from Earth is the same size as  the moon. About 30 arc minutes or 1/2 degree. Never look at the sun with bare eyes, but on a foggy day, where you can just make out the sun's disk you can see this. you can also use a welding lens of no less than a number 12 shade. Also be very careful if using a glass lens (the better quality lenses are all glass) because if the lens becomes too hot from the sunlight it can break. Don't look at the sun very long with this method but you can look long enough to see the disk.

I'm not sure but for the sun to look like a point source of light I think you need to be a parsec away, which is over 3 light years.
« Last Edit: 22/12/2009 06:19:36 by Eric A. Taylor »
 

Offline Eric A. Taylor

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Why does an airplanes shadow become a bright spot?
« Reply #6 on: 22/12/2009 06:20:41 »
Diffraction can cause a bright spot in the centre of a shadow ... http://en.wikipedia.org/wiki/Arago_spot


http://www.sciencephoto.com



Sorry, don't think that's right. A Cessna 177rg is not round and the sun is not a point source of light.

"In optics, an Arago spot is a bright point which appears at the center of the shadow of a circular object in light from a point source."


I think that is on the right tract actually.  I don't know that one could see well enough to resolve the shape of a point source of light from altitude, but the contrast difference may be noticeable and your brain interprets this as a bright spot.

I'm not sure that's right. From the altitude of 2000 the sun looks the same as it does on the ground. In fact the sun is so far away you have to get several million miles closer for it to look any bigger. The disk of the sun, as seen from Earth is the same size as  the moon. About 30 arc minutes or 1/2 degree. Never look at the sun with bare eyes, but on a foggy day, where you can just make out the sun's disk you can see this. you can also use a welding lens of no less than a number 12 shade. Also be very careful if using a glass lens (the better quality lenses are all glass) because if the lens becomes too hot from the sunlight it can break. Do not try to use a quick change lens as they don't change from sunlight. Don't look at the sun very long with this method but you can look long enough to see the disk.

I'm not sure but for the sun to look like a point source of light I think you need to be a parsec away, which is over 3 light years.
 

Offline RD

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Why does an airplanes shadow become a bright spot?
« Reply #7 on: 22/12/2009 07:24:12 »
Diffraction occurs whether or not the light source is a point, and whether or not the obstruction is a disc.

BTW the apparent bright spot where the plane shadow was could be physiological ...
http://en.wikipedia.org/wiki/Afterimage#Negative_afterimages
« Last Edit: 22/12/2009 07:46:28 by RD »
 

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Why does an airplanes shadow become a bright spot?
« Reply #7 on: 22/12/2009 07:24:12 »

 

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