- #1
Reflection from double glazed window
- B
- Thread starter franchester
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- #2
A.T.
Science Advisor
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The glass is not perfectly flat. It deforms within the frame, due to thermal expansion, windpressure, etc.franchester said:
- #3
Ibix
Science Advisor
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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.
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.