When flying my Cherokee 235 over the African Bushvelt at a certain height, I have noticed that where the shadow of the aircraft should be on the ground it is absent, but in its place is a bright patch of light. This is obviously a phenomenon caused by the suns rays being refracted around the edge of the aircraft and at a certain height the refracted light is focused close together on the ground. Taking this effect further, it would be obvious that a circular object would be able to get all the refracted light into a smaller spot on the Earth. This effect can also be notice, albeit on a much smaller scale early in the morning where a bright ring can be seen around your head on your shadow cast on the ground. What is this effect called. Has it ever been used to focus light on a single spot such as solar water heating or other uses.
It isn’t due to light being bent around the aircraft (it would be diffraction rather than refraction.) The effect is only visible to you in the aircraft, and is due to light being scattered back directly to you along the path the light took from the sun. An observer anywhere other than in the aircraft would see a perfectly normal shadow on the ground. You might also see the shadow surrounded by light, or if there are enough water droplets in the air, surrounded by a complete rainbow effect - a glory.
Is it an Arago spot ?
No, this bright spot is also noticed by observers on the ground. I looked at Arogo Spot on Wiki but it shows a large shadow around the spot. This spot has no shadow at all but the brilliance is amplified incredibly. i would love to do tests to see if it is both the light that can be focused as well as the infra red spectrum which could then be used for heating purposes.
There are several effects at play here - one of them (the simplest) is that the shadows of objects near your shadow are hidden behind the objects themselves - because you’re viewing those objects from the same direction as the source of illumination. This effect is really pronounced if the ground is covered with wheat stubble.
But you say the effect is noticeable by an observer on the ground - that’s kinda puzzling - I mean, there are other components to the effect that would make it so, but I’m surprised a ground based observer can really make any careful observations about a shadow that passes by very quickly.
Are you sure it’s not just glare/reflections from your cockpit and/or plane? How does the patch of light change when you maneuver?
The bright spot follows the aircraft wherever it goes and over different type of terrain. The only thing that makes it disappear is a change of altitude over the terrain. The effect is very noticeable at one certain height above ground.
I think that I must start experimenting with different size discs and see what the ratio is of size versus altitude with the sun size being the only constant.
I am surprised that no other pilots have reported this phenomena.
There are a number of pilots on the dope, but I suspect you’re thread title hasn’t lured many of them here. You might try “FLYING question and OPTICAL illusion” or something like that. It’s all about the building the buzz.
Sailplane pilot here. Observed this many times. I have also observed that there is only one bright spot when I am on tow, so at the same altitude, and 200’ from another aircraft. I can see the shadow of the towplane, and my own, but the bright spot will only be around my own shadow.
The ground has lots of things that cast shadows. Grass, brush, vehicles, buildings, etc. For places on the ground where you are directly between that spot and the sun, you can’t see any of those shadows, because the object is in the way, and that object is getting full sun, so it all looks bright. I have also observed that there is no bright spot when the ground is bare, such as parking lots. Asphalt parking lots DO tend to throw off thermals, so there is good reason to seek them out when getting low.
That makes sense, and is what I was going to suggest, but the OP seems to be claiming there is no shadow. Presumably above a certain height, the shadow gets too small to see but the bright spot is still visible?
None of which would explain observers on the ground being able to see it. I suspect Zep Tepi is mistaken about that part.
Have a look at this page and scroll halfway down to the section headed Opposition effect. Is this what you mean? As noted above, it’s caused by the shadows of objects being hidden by the objects themselves. Presumably at these altitudes, the shadow of the plane is too small to see.
If there was a spectral component, I’d say it was a Glory, otherwise some form of Heiligenschein, sylvan shine, oppositional surge or similar.
But I really came in here to say that it’s spelled Bushveld.
I’ve edited the thread title to make it clearer.
Not just too small to see, but the the umbra (dark part of shadow) only exists at all for a certain distance.(the distance where the angular size of the object casting the shadow is larger than the angular size of the sun) Yes, the bright spot is still obvious even when high enough not to have a umbral shadow. It is just less obviously associated with being exactly opposite the sun. The bright spot normally makes it difficult/impossible to see your penumbral shadow from an airplane.
I have also spent a lot of time at airports, and had airplane shadows pass over me lots of times, and in no case did the ground ever get brighter, only darker for a brief instant. After a while you learn not to look up and see who that was, because of course you will be looking right into the sun.
Oh yes, all the pilots I know refer to this effect as “the bright spot”.
Colophon has it right. The Opposition Effect is undoubtedly what you’re looking for. The other effects aren’t relevant.
Heiligenschein requires dew droplets suspended on the hairs of grass, or something similar. The glory occurs when you have light reflected from tiny droplets and retrorflecting while interfering with each other. (THe Spectre of the Brocken is the same effect, seen from a mountaintop on a fog-filled layer below). The glory requires that you be above a layer of fog or cloud.
You don’t have to be in an airplane high above the ground to see this effect – you can see it on the ground or a hillside not far away from you. See M. Minnaert’s classic text The Nature of Light and Color in the Open Air, which gives examples.
You the man, man.
Oh, and it’s not an Arago Spot (Poisson Spot). That demands a round object interposed between the light source and the “screen”. I’ve eliminated a similar (unwanted) effect by changing the outline of the object from circular to square. A plane’s shadow definitely isn’t round. On top of which, the Arago/Poisson spot is an amazingly timy effect – you would’t get the large apparent bright spots you see from this effect. And, as Ze Tepi observes, there is a dark ring around the Arago/Poisson spot.
As Francis Vaughn mentioned in the very first response, it is indeed the glory (a.k.a. the heiligenschein or gegenschein in various contexts). Look at the Wikipedia article Francis linked and you’ll see more than one picture taken from an airplane.
I love watching the Glory from my airplane window, especially watching from takeoff as the shadow of the plane gets smaller and smaller until it is surrounded by the glory and eventually disappears leaving only the glory behind. Many materials (not just spherical drops) exhibit a pronounced retroreflection, i.e. reflect light directly back to the source. Thus, if you are far enough away for the shadow of your head or the plane to be smaller than the angular range of the retroreflection peak, you will see the glory.
My favorite example is the retroreflection of the Moon’s surface, evident in the clearly visible glory in the reflecting visors of the astronauts. In the reflection, you can often see the astronaut’s shadow, with a glory surrounding the shadow of his head.
Here is an example from the cover of Life Magazine:
I had noticed the opposition effect long ago when looking at Apollo 11 photographs.
In particular, this one: http://www.hq.nasa.gov/alsj/a11/AS11-40-5882.jpg
I’ve also seen it from planes. I think the reason the OP doesn’t see a shadow in the middle of the bright spot is that there is no spot on the ground whose view of the sun is fully blocked, or even mostly blocked, by the plane. As an object gets farther and farther from the shadow it casts, the shadow gets smaller because the Sun is not a point light source.
As the plane gets higher, the umbra (the part of the shadow that’s completely shielded from the Sun) shrinks and then disappears, but the penumbra (the partial shadow, where some of the Sun’s light is blocked) gets bigger. But while it’s getting bigger, it’s also fading. A high-flying airplane has a huge penumbra, but the decrease in light anywhere in that huge penumbra is so slight that it can easily go unnoticed.