Odd sunlight reflections off glass windows

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tomandlu
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Odd sunlight reflections off glass windows

Postby tomandlu » Fri Dec 22, 2017 3:05 pm UTC

The attached picture shows the patterns created by sunlight being reflected off of windows behind the camera onto the walls of some flats that are being built on vacant land at the end of our garden. What am I seeing? I'm only showing a couple of the reflections here, but our house is part of a terrace, and the patterns are visible for almost all the applicable houses.
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Re: Odd sunlight reflections off glass windows

Postby measure » Fri Dec 22, 2017 4:31 pm UTC

tomandlu wrote:The attached picture shows the patterns created by sunlight being reflected off of windows behind the camera onto the walls of some flats that are being built on vacant land at the end of our garden. What am I seeing? I'm only showing a couple of the reflections here, but our house is part of a terrace, and the patterns are visible for almost all the applicable houses.

Perhaps the window panes are slightly concave? Flat rectangular edges and a depressed center might cause an effect like that.

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Re: Odd sunlight reflections off glass windows

Postby tomandlu » Fri Dec 22, 2017 4:36 pm UTC

measure wrote:Perhaps the window panes are slightly concave? Flat rectangular edges and a depressed center might cause an effect like that.


I assume it's something like that, but either all glass is like that (and I've just never had the opportunity to see it before), or there's something very odd about our street...
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Re: Odd sunlight reflections off glass windows

Postby measure » Fri Dec 22, 2017 5:38 pm UTC

tomandlu wrote:I assume it's something like that, but either all glass is like that (and I've just never had the opportunity to see it before), or there's something very odd about our street...

If it is this effect, it would only be visible on surfaces at a range of distances determined by the focal length of the reflector. Too close, and you'd see a less distorted rectangle. Too far, and the image would be too large and faint to see.

It's not uncommon for me to see distorted reflections in windows if I'm far enough away from them.

Many newer windows are double-paned with a sealed gas between. Variations in air pressure could cause fairly uniform distortion for multiple windows, especially if they we of the same type and recently installed.

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Re: Odd sunlight reflections off glass windows

Postby tomandlu » Fri Dec 22, 2017 6:01 pm UTC

measure wrote:If it is this effect, it would only be visible on surfaces at a range of distances determined by the focal length of the reflector. Too close, and you'd see a less distorted rectangle. Too far, and the image would be too large and faint to see.

It's not uncommon for me to see distorted reflections in windows if I'm far enough away from them.

Many newer windows are double-paned with a sealed gas between. Variations in air pressure could cause fairly uniform distortion for multiple windows, especially if they we of the same type and recently installed.


Well, it's a fairly old terrace (1920s I'd guess), so I doubt the glass is of the same type or installation date between one house and another. However, I like the idea that the wall the reflections are being projected on is at the optimal focal length. The wall and the windows run basically parallel.

It's not the distortions themselves that bother me, so much as their crystal-like precision, and the fact that I can't seem to find anything on this phenomena (so far).
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Re: Odd sunlight reflections off glass windows

Postby Soupspoon » Fri Dec 22, 2017 6:18 pm UTC

There's a similar (if slightly differently configured) multi-reflection effect on a street I often am at during sunset when the suitably-facing side of the street projects the low-angle rays back against the over-street building faces in shadow.. The buildings are all semi-detached private houses with individually-replaced glazing (there might have been some "your neighbour just had their windows replaced by us, why not also give us a try?" effect, that might have stacked the suppliers/installers as being the same).

As well as predictably similar small concavity of the pane, I also put it down to edge-diffraction, kind of like a slit experiment. But I never did do the maths to see if the effects survived into the macro-scale involved.

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Re: Odd sunlight reflections off glass windows

Postby eSOANEM » Sat Dec 23, 2017 12:19 am UTC

Yeah, that'll be a reflection off some sort of concave object. The name for an image like this is a caustic (more info at the link)
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Re: Odd sunlight reflections off glass windows

Postby tomandlu » Sat Dec 23, 2017 5:57 pm UTC

eSOANEM wrote:Yeah, that'll be a reflection off some sort of concave object. The name for an image like this is a caustic (more info at the link)


Yeah, I used to mess around with Povray and photons, so I'm familiar with the general phenomena. It's the specificity of this that's surprising me. The distortion seems to be in the glass, and is very structured, yet each pane seems to throw a different geometrical shape. I guess I'm just surprised I can't find similar images on Google or specific references to it.

When I first saw it, I assumed that there bathroom mirrors were involved, or sun-catchers or something, yet it appears to be just sheets of glass.
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Re: Odd sunlight reflections off glass windows

Postby jewish_scientist » Sun Dec 24, 2017 3:33 pm UTC

measure wrote:Variations in air pressure could cause fairly uniform distortion for multiple windows...

Do you think that the differences in temperature between the outside and inside could cause the differences in pressure?

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Re: Odd sunlight reflections off glass windows

Postby tomandlu » Sun Dec 24, 2017 4:31 pm UTC

jewish_scientist wrote:
measure wrote:Variations in air pressure could cause fairly uniform distortion for multiple windows...

Do you think that the differences in temperature between the outside and inside could cause the differences in pressure?


I don't think it's much affected by temperature differentials - at least it isn't limited to a particular season iirc.
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Re: Odd sunlight reflections off glass windows

Postby measure » Tue Dec 26, 2017 5:22 pm UTC

jewish_scientist wrote:
measure wrote:Variations in air pressure could cause fairly uniform distortion for multiple windows...

Do you think that the differences in temperature between the outside and inside could cause the differences in pressure?

I was referring to a pressure difference between the atmospheric pressure (on both sides of the glass) and the pressure of the sealed pocket between the panes.
If there was a static pressure differential between the two sides, it should pretty quickly equalize. However wind or the HVAC system of the building could maintain a pressure difference.

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Re: Odd sunlight reflections off glass windows

Postby tomandlu » Wed Dec 27, 2017 8:05 am UTC

measure wrote:
jewish_scientist wrote:
measure wrote:Variations in air pressure could cause fairly uniform distortion for multiple windows...

Do you think that the differences in temperature between the outside and inside could cause the differences in pressure?

I was referring to a pressure difference between the atmospheric pressure (on both sides of the glass) and the pressure of the sealed pocket between the panes.
If there was a static pressure differential between the two sides, it should pretty quickly equalize. However wind or the HVAC system of the building could maintain a pressure difference.


Ah - I'd be surprised if all these windows were double-glazed, but I suppose it's possible.
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