Reflection from double glazed window

[franchester]

TL;DR Summary

Sorry not technical minded

Can anyone explain why we are having this strange reflection on our outside wall, from the sun hitting our bedroom window?

 

[A.T.]

Can anyone explain why we are having this strange reflection on our outside wall, from the sun hitting our bedroom window?

The glass is not perfectly flat. It deforms within the frame, due to thermal expansion, windpressure, etc.

 

[Ibix]

As A.T. says, the glass isn't quite flat. From the pattern, I'd think it's slightly concave and taller than it is wide.

Imagine dividing the window up like this:

If the glass is slightly curved inwards, the triangle at the top is slightly tilted down, the one at the bottom is slightly tilted up, and the side ones are slightly tilted towards each other.

If you think what the slightly tilted down bit does with reflected light, it slightly displaces the reflection downwards - so the triangle of light reflected by the top triangle slightly overlaps the very top of the reflections from the side pieces. Similarly, light reflected from the bottom triangle is slightly displaced upwards and overlaps the reflections from the very bottom of the side pieces. And the reflections from the side pieces are slightly displaced inwards and overlap each other in the middle. The overlapped areas get double the light of other areas, so are double-brightness compared to the rest of it. That's the origin of the bright X.

The reality is a bit messier than my description because the glass is (slightly) curved rather than folded sharply along neat lines, which is probably how come you have the sort of distorted circular reflection from what I expect is a rectangular window.

 

[Gleb1964]

I think that is hermetically sealed glass pocket. Air between double glass has a different pressure to the ambient air producing glass sheet deformation. I observe similar effect in my house, the light reflected from window is focused on the wall of nearby house. Tree windows in a row with the same effect.

Hermetic Double glazing (DVH) or insulated, are panels composed of two glass sheets, hermetically sealed by thermoplastic tape, existing between both layers a chamber of dehydrated air that provides greater acoustic and thermal insulation compared to a monolithic glass.

 

[Gleb1964]

Reflection from the windows that I observe daily

 

[sophiecentaur]

Try pushing against the glass (from inside or outside) and note any change in that pattern.

 

[marcusl]

These look like caustics, which are loci (curves) where light rays are overlaid after being reflected from or diffracted through curved objects. See, e.g., https://en.m.wikipedia.org/wiki/Cau...tics, a caustic or,of rays on another surface

 

[Gleb1964]

Try pushing against the glass (from inside or outside) and note any change in that pattern.

Catching a nice sunny day when reflection structure is visible in a greater details, I took a new photo. The window is a triple-glazed window, it gives three reflections. Pushing by finger the inside glass gives visible effect on the outside ring in reflection. That suggests that the air pressure inside pocket is below ambient and windows have a slight sag inside and the middle glass is about flat.
The reflection from inside glass have a slightly different colour, there should be some sort of coating, probably preserving the near infrared radiation(?). Potentially, I can measure the transmission curve with an mid-IR spectrometer (not at high priority those).