Double Glazed Window Reflection

[brbrown]

I hope this is not a stupid question but I attach a photo of a reflection of a square double glazed window showing on a wall opposite the window. Could someone please explain to me what is happening here. I presume the outer circular reflection is showing the extent of the suns rays and the cross is a separate reflection of both panes of glass. Is that so and if so why does it show as a cross?

 

[berkeman]

Pretty cool question. Can you clarify -- this is a reflection off the window that is hitting an exterior wall outside the window? Can you post a sketch of the position of the sun, window and exterior wall?

Or is this an image of the Sun formed on an interior wall after the Sun's rays pass through the window? If so, a sketch would still help. Thanks.

 

[sophiecentaur]

why does it show as a cross?

I think you need to provide a detailed diagram of the arrangement if you want a credible explanation. You seem to be assuming that the pattern actually appears on the wall but I don't think you would see that pattern on the wall if you went outside. That would have to involve some sort of projection arrangement (curved mirror or lens). So far, it looks to me as if it's a virtual image but you could solve this by moving your head from side to side and seeing if the cross appears to move over the wall.

The ring could be because of slight surface roughness of the glass (similar to Sundogs and halos which you can see when looking through clouds of water vapour or ice crystals). Perhaps the cross is due to slight differences in curvatures of the glass faces.

I remember seeing a similar cross shaped image of a star when subjected to gravitational lensing. I think it was on PF, but I couldn't find it from a brief PF search.

 

[Baluncore]

A double glazed window probably does not have a fly screen, so maybe rule out a Moire pattern.

Tempered or toughened glass is sprayed with air or oil while cooling. That results in thickness, stress, polarisation and surface variations. Maybe the pattern reflected results from the pattern of cooling, being resolved by the internal reflections.
https://en.wikipedia.org/wiki/Tempered_glass#Manufacturing

 

[Keith_McClary]

If a single glazed window was perfectly flat the reflection would be a rectangle (assuming the brick wall is parallel). If it is slightly concave on the outside the reflection will be deformed into a shape like the cross. If you can push on the center outside you will probably see a change in the cross shape. If one of the shapes is from the inner pane, pushing on that should change the shape.

 

[Blue Chuzzle]

There are different types of double glazing, some with air or a gas and others with a Vacuum like a thermos flask. The vacuum causes the panes to flex inwards slightly and this causes the pattern you see here.

 

[Baluncore]

@Blue Chuzzle, welcome to PF.
Do you have a reference, or how did you find that explanation?

 

[russ_watters]

Is this a reflection or refraction?

 

[sophiecentaur]

If a single glazed window was perfectly flat the reflection would be a rectangle (assuming the brick wall is parallel). If it is slightly concave on the outside the reflection will be deformed into a shape like the cross. If you can push on the center outside you will probably see a change in the cross shape. If one of the shapes is from the inner pane, pushing on that should change the shape.

I think this requires some explanation from you. What light path(s) are you using for this effect?

Is this a reflection or refraction?

It could almost be diffraction?

 

[brbrown]

Hello Everone. Thanks for your replies. Sorry about the delay in responding only I didn't get a notification of any replies by email as I elected to do. I list my answers to some of your questions.
1) Berkeman, the reflection is on the wall opposite the window. The sun was approximately 20 degrees to the right of the reflection past the wall and was low down in the sky towards sunset. When I opened the window, the reflection obviously moved to a new position.
2) Sophie, not sure how I can describe the situation any clearer. As I said above, the reflection was on the wall. I could see it there when I went outside. I don't understand. Why is that a puzzle? Glass reflects like mirrors do. I did wonder if the glass was distorted and slightly curved.
3) Blue, I didn't think to try pressing the glass to see any shape change in the reflection. I'll try that if I see the reflection again.
4) Russ I suppose that it is technically a refraction in that the light is passed from one medium to another but I think of it as a reflection.
I hope that answers the questions satisfactorily. Thanks again for the responses. Best wishes. Brian

 

[sophiecentaur]

2) Sophie, not sure how I can describe the situation any clearer.

One possible reason that you have had such a variety of answers is probably that you really haven't given us a good idea of the actual setup. It's obvious to you, 'cos you were there but some important details are missing for us. When you were actually outside, looking directly at the wall (not through the window) did the pattern look any different from the view from indoors?? Was the sun low or high in the sky (perhaps just peeping over the wall at the window? How far from the wall was the window? Was the light path from the Sun obstructed in any way. Was the sky actually clear?

I presume the outer circular reflection is showing the extent of the suns rays

I'm really confused what you could have meant by this. The Sun radiates in all directions, of course, so what would the "extent" mean?

 

[Pratyeka]

If a single glazed window was perfectly flat the reflection would be a rectangle (assuming the brick wall is parallel). If it is slightly concave on the outside the reflection will be deformed into a shape like the cross. If you can push on the center outside you will probably see a change in the cross shape. If one of the shapes is from the inner pane, pushing on that should change the shape.

Depends on the distance between the reflecting surface and the wall, the shape of the reflection will be circular (it's an image of the sun) after some distance, no matter what the shape of the mirror is.

 

[sophiecentaur]

If a single glazed window was perfectly flat the reflection would be a rectangle

That's true but the size of that rectangle would be a rectangle that's larger than the window. Same as if the Sun were shining through a doorway; a larger image of the doorway would appear on the opposite wall. The same rules apply as for shadows - always an image bigger than the obstruction and not 'focussed' any way.

Depends on the distance between the reflecting surface and the wall, the shape of the reflection will be circular (it's an image of the sun)

This, also doesn't make sense in this context. Whilst it's true than a convex lens or concave reflector can produce a real projected image, the distance of the wall from the window and / or the probable curvature (giving a focal length) makes this unlikely - UNLESS, and I have made this point twice already, the reflecting window is a long distance from the wall but this information has not been given yet so we can only speculate.
In all Physics, the numbers count and they govern the result in any situation like this. What is the actual layout?

 

[Pratyeka]

In the case of a flat mirror (or flat window) reflecting the sun, the effect is the same as pinhole camera or a pinhole in a shoebox used to observe solar eclipse. This is something I've learned from experiments when I was 10 years old (ages ago last millennium).

also take a look at this article: https://windowfilmforturf.com/pages/how-to-stop-window-reflection-1

 

[Baluncore]

also take a look at this article: https://windowfilmforturf.com/pages/how-to-stop-window-reflection-1

That shows the same effect as in the OP. "Sunlight Reflected from Double-Paned Low-E Windows, and Damage to Vinyl Siding and Other Materials"

 

[sophiecentaur]

the effect is the same as pinhole camera or a pinhole in a shoebox

You have not remembered your elementary optics quite correctly. There is no equivalence between the pinhole and a large flat reflector images. If you had taken your shoebox and made two pinholes in it, the image would have been two Suns. n pinholes produce n Suns, side by side and a large square hole An infinite number of pinholes produces an image that's square and of a similar shape as the hole and larger, according to the ratios of hole size, projection throw etc. ). Without some curvature, you can get no real image formed. Why would anyone use a lens if a big hole would do the same job?
As I said before, the mirror acts in the same way as a hole of the same size. If you stand, looking through the window at he Sun , you will see the sun as you move around the room until its gets to the edge of the window. A small Penumbra region and then you don't see it. Ditto for the reflection from a large window. It's elementary shadow geometry.

 

[sophiecentaur]

also take a look at this article: https://windowfilmforturf.com/pages/how-to-stop-window-reflection-1

That's a good image, repeating the effect in the OP. I can't think there's any doubt that it's a focusing effect of a curved reflector. Although they still don't mention the actual distance between window and wall we now know we're talking in terms of across a street (say 50m). That must be similar to the curvature of one of the glass faces (it could be either) to form that basic part -focused spot. Pressure differences due to heating of the units on a hot day could make the bowing worse. The actual shape of the glass wouldn't be spherical, the curvature would be different along the diagonals and for the verticals and horizontals. That's astigmatism, which produces that sort of distortion.
I'd even go as far as to say that the ring could be due to a double reflection and two radii at work.

One thing surprises me and that is it's quite a rare effect, when you think of the millions of double glazed panes all over cities and all at different, ever changing angles relative to the Sun. Perhaps it's restricted to one make of unit.

A 'good' thread, I think.

 

[brbrown]

Sorry everyone. I am becoming bewildered with all the variation of answers. I am beginning to wish I had not asked the question. You have all given me a lot of interesting comments and I thank you all for your help, interest and perseverance.

 

[Baluncore]

Dear Bewildered.
Think of it this way.

Number the surfaces, 1 is external outside glass, 2 is coated with metal film, 3 and 4 are the inside pane. The inner surface =2, of the outside pane is coated with a metal film which makes it partially reflective. The metal particles are probably of a size that selectively reflect long wavelength IR, while allowing shorter wavelength visible light to pass.

The double glazed window is filled with a low pressure dry gas to protect the metal film and prevent internal condensation. That pulls the two glass sheets slightly closer together near the middle, which curves the panes.

The outer pane with surface 1 and the more reflective metalised surface 2, is slightly concave, so it tends to focus the sun into the centre of the reflection on the wall.

Because the window frame is rectangular, there is a boundary condition that causes the pane to be flatter at the straight frame sides. Therefore a slight trough forms from the four corners to the centre. Those diagonal corner troughs reflect the light that forms the 'X' on the wall.

Meanwhile the inner pane is convex which makes it divergent. External light, reflected back from surfaces 3 and 4 will be attenuated twice by having to pass through the coated layer 2 twice. That divergent rear reflector will tend to light the whole area of the wall. The flatter areas of surfaces 3 and 4, constrained by the straight frame, may be generating the faint outer circle.

 

[Keith_McClary]

The vacuum causes the panes to flex inwards slightly

Atmospheric pressure is 10 tonnes per square meter.

https://www.gowercroft.co.uk/news/what-is-vacuum-glazing/

 

[Orthoceras]

Similar cross and circle reflections occurred at a university building in Germany. Physicists investigated the phenomenon and published their results in the Eur J Phys (2014), with a lot of interesting images. [pdf]

From their article:

• The increased use of double pane windows in recent decades has led to an increase of observations of everyday life sightings of reflection images of strange and varying geometrical forms.
• The best conditions to observe the phenomena are a low shining sun and a street running perpendicularly to the direction of the sunlight, i.e. near normal incidence of light onto the window.
• For a given location, the phenomenon can be observed for a couple of hours each day for a period of several weeks within a given time of the year.
• If the projection wall distance changes, it may well be that the same window results in crosses for nearby projection and round spots for more distant projection.
• The concave, focusing pane gives rise to an extended cross-like structure whereas the convex defocusing pane yields the outer rhombic-like feature.
• Applying a force to the inside of a double pane window (manually pushing against the center of one pane) changes its inside pressure (not a vacuum) and therefore the geometry of the outside pane. The originally in focus image changes into an out of focus projection on the wall while applying the force.

 

[marcusl]

The bright arcs in the photo in sophiecentaur’s link are caustics that result from reflection off a curved or distorted surface. One wouldn’t see these if the window were completely flat.

 

[Keith_McClary]

From their article:

Figure 6.Moving caustic shape due to tilting of a window, i.e. rotation around a horizontal axis ...

I have a crank-out window like that, so I can (hypothetically ) adjust the beam to zap passers by.

 

[sophiecentaur]

The bright arcs in the photo in sophiecentaur’s link are caustics that result from reflection off a curved or distorted surface. One wouldn’t see these if the window were completely flat.

You wouldn't get a focused image either - just a massive bright disc with penumbra.

I have to wonder how much 'vacuum glazing' you can find amongst your average windows. The internal supports would be very visible and I can't say I've ever noticed their presence in the units I've seen. (Excellent for some applications, no doubt.)

 

[Baluncore]

The internal supports would be very visible and I can't say I've ever noticed their presence in the units I've seen.

Vacuum Insulated Glass is not yet widely used. The spacers, on a 20 mm grid can be made from stainless steel or glass, with a thickness of 0.2 mm and diameter 0.6 mm.
I think you could ignore them in most applications as they are out of focus and smaller than the pupil of your eye.

 

[sophiecentaur]

Vacuum Insulated Glass is not yet widely used. The spacers, on a 20 mm grid can be made from stainless steel or glass, with a thickness of 0.2 mm and diameter 0.6 mm.
I think you could ignore them in most applications as they are out of focus and smaller than the pupil of your eye.

Good for some things but flare could be a real problem for a picture window at the sunny side of a house.