Circular effect of light through trees - Is there a name for this?

Ever looked at the moon or a streetlight at night through tree branches (especially after it rains) and notice how the reflecting branches seem to “curve” around the light source? I’ve also seen this looking at the reflection of a light on a shiny but scratched surface like a finished schooldesk.

So, is there a name for this effect?

No clue about the name of the effect, but I’ll try to remember to check it out tonight, Mixolydian. I’ll have to rely on the moon as the light source. We haven’t many street lights in my town - maybe 5 or so. And no rain in sight.

Sounds interesting, though.

Is your name related to a form of musical scales?

It is!

http://en.wikipedia.org/wiki/Mixolydian_mode

The more branches/twigs, the better. Maybe get out the hose? :slight_smile:

I should mention that I first noticed this as a teen along with a friend while under the (albeit weak) influence of a hallucinogen. It’s still there without “help” if you look for it.

To clarify the OP a bit more, I think it’s similar to what Van Gogh was trying to
duplicate in “Starry Night”.

Could be, rabbit… :wink: <edit> I see you already confirmed…the byproduct of an influential music theory teacher and the mode’s connection to the blues, my genre of choice</edit>

Minnaert (naturally) discusses the effect in his classic The Nature of Light and Colour in the Open Air, but doesn’t mention a specific name for it. I doubt it has a scientific name, though some artist’s manual might have given it a name at some point.

Minneart’s book is filled with detailed, yet clear, explanations of all sorts of such everyday effects. Highly recommended.

I think this is a large-scale example of chatoyancy.

Why?

I don’t remember observing this effect. Is it something that can be captured on photos?

Isn’t it just simple diffraction?

I don’t think so… On looking up chatoyancy, it looks like it depends on having some sort of parallel grain in the material, while this effect depends on having linear features running in many different random directions.

In case anyone is unclear, in the OP’s effect, there are branches or scratches going every which way; it’s just only those which would appear to encircle the light source which get highlighted.

ETA:

No, it’s a purely ray-optics effect, independent of wavelength. And there’s no reason it wouldn’t show up in a photo, so long as you didn’t use a flash.

In that case, could anybody be so kind and provide a photograph of this effect? I am curious of what you all seem to have observed.

Courtesy of Google Book Search, here’s the (not wildly clear) photo of the effect from the Dover edition of Minnaert.

Bonzer,

I think I’m going to have to check that book out. :wink:

Thank you, all - this one has bugged me for ages, and it ain’t the kind of thing you bring up in any average conversation. The Dope does it again!

I also wondered what, if any relationship there was to the phenomenon and the dual particle/wave nature of light, but hey, this is more than I bargained for as it is. Thanks again.

I’m thinking the word is Anisotropic. I know the effect you’re talking about, and it’s basically an anisotropic effect of random etchings at all angles in a surface, rather than a radial etching like that on a brushed aluminum surface.

ETA: could be Isotropic. Looking into it…

Well, my best guess for now, is random anisotropic specularity.

>I don’t think so… On looking up chatoyancy, it looks like it depends on having some sort of parallel grain in the material, while this effect depends on having linear features running in many different random directions.

Chatoyancy depends on parallel grains scattering light in the plane perpendicular to the grain and containing the source. The effect described in the OP is scattering by the sufficiently nearly parallel fraction of all the randomly oriented structures in this same configuration. That is, all the branches are random, but the reflecting subset in a particular region are all nearly parallel and tun perpendicular to the plane containing the reflecting spots, the sun, and the observer. I say in a particular region because as you travel around the line of sight to the sun, their angle changes.
So, I think the effect the OP notices is that, of all the branches, those that are at the right angle are emphasized by reflecting, which is chatoyancy at work; and, this is on a large scale a circular grain effect because you are rotating the plane in which the branches are made reflecting.

An easy place to spot it is on car paintwork when the sun reflects off it. The tiny random scratches in the paintwork from washing it show up in a circle around the image of the sun.

Here’s an example (using a lamp rather than the sun).

Edit: and here is another which shows the effect more clearly.

With regard to trees, isn’t this an example of fractals? So that when the tree is in leaf it captures the maximum amount of sun?

Is this effect related at all to how you can look down at the ground under a tree during an eclipse and see multiple crescents on the ground?

*** Ponder

No, what you’re talking about is related to the Bokeh Effect, I believe. Entirely different than what the OP is asking about. See Colophon’s pics.